start/pause/stop: Readd 'generateHandler' helper

This reintroduces code which was removed in 481c580.
The Block_copy/Block_release calls from this helper seem to be needed on my
x86_64 macOS11 machine. Without this, example/linux/virtualization segfaults
in startWithCompletionHandler.

This fixes this ASAN error:
=================================================================
==56003==ERROR: AddressSanitizer: requested allocation size 0x53cb4e83f8b48 (0x53cb4e83f9b48 after adjustments for alignment, red zones etc.) exceeds maximum supported size of 0x10000000000 (thread T6)
    #0 0x43f0400 in wrap_malloc+0xa0 (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x44400)
    Code-Hex#1 0x7fff205753ba in _Block_copy+0x5e (libsystem_blocks.dylib:x86_64+0x13ba)
    Code-Hex#2 0x7fff6f594307 in Base::BlockPtr<void (bool)> Base::BlockPtr<void (bool)>::from_callable<-[VZVirtualMachine startWithCompletionHandler:]::$_13>(-[VZVirtualMachine startWithCompletionHandler:]::$_13)::'lambda'(void*, bool)::__invoke(void*, bool)+0xb37 (Virtualization:x86_64+0x24307)
    Code-Hex#3 0x43ef5fa in __wrap_dispatch_async_block_invoke+0xca (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x435fa)
    Code-Hex#4 0x7fff20691622 in _dispatch_call_block_and_release+0xb (libdispatch.dylib:x86_64+0x2622)
    Code-Hex#5 0x7fff20692805 in _dispatch_client_callout+0x7 (libdispatch.dylib:x86_64+0x3805)
    Code-Hex#6 0x7fff206985e9 in _dispatch_lane_serial_drain+0x25d (libdispatch.dylib:x86_64+0x95e9)
    Code-Hex#7 0x7fff206990ac in _dispatch_lane_invoke+0x16d (libdispatch.dylib:x86_64+0xa0ac)
    Code-Hex#8 0x7fff206a2c0c in _dispatch_workloop_worker_thread+0x32a (libdispatch.dylib:x86_64+0x13c0c)
    Code-Hex#9 0x7fff2083945c in _pthread_wqthread+0x139 (libsystem_pthread.dylib:x86_64+0x345c)
    Code-Hex#10 0x7fff2083842e in start_wqthread+0xe (libsystem_pthread.dylib:x86_64+0x242e)

virtualization.m

@@ -1005,47 +1005,60 @@ bool requestStopVirtualMachine(void *machine, void *queue, void **error)
     return queue;
 }
 
+typedef void (^handler_t)(NSError *);
+
+handler_t generateHandler(void handler(void *, char *))
+{
+    handler_t ret;
+    @autoreleasepool {
+        ret = Block_copy(^(NSError *err){
+            virtualMachineCompletionHandler(handler, err);
+        });
+    }
+    return ret;
+}
+
 void startWithCompletionHandler(void *machine, void *queue, void *completionHandler)
 {
     if (@available(macOS 11, *)) {
+        handler_t handler = generateHandler(completionHandler);
         dispatch_sync((dispatch_queue_t)queue, ^{
-            [(VZVirtualMachine *)machine startWithCompletionHandler:^(NSError *err) {
-                virtualMachineCompletionHandler(completionHandler, err);
-            }];
+            [(VZVirtualMachine *)machine startWithCompletionHandler:handler];
         });
+        Block_release(handler);
     }
 }
 
 void pauseWithCompletionHandler(void *machine, void *queue, void *completionHandler)
 {
     if (@available(macOS 11, *)) {
+        handler_t handler = generateHandler(completionHandler);
         dispatch_sync((dispatch_queue_t)queue, ^{
-            [(VZVirtualMachine *)machine pauseWithCompletionHandler:^(NSError *err) {
-                virtualMachineCompletionHandler(completionHandler, err);
-            }];
+            [(VZVirtualMachine *)machine pauseWithCompletionHandler:handler];
         });
+        Block_release(handler);
     }
 }
 
 void resumeWithCompletionHandler(void *machine, void *queue, void *completionHandler)
 {
     if (@available(macOS 11, *)) {
+        handler_t handler = generateHandler(completionHandler);
         dispatch_sync((dispatch_queue_t)queue, ^{
-            [(VZVirtualMachine *)machine resumeWithCompletionHandler:^(NSError *err) {
-                virtualMachineCompletionHandler(completionHandler, err);
-            }];
+            [(VZVirtualMachine *)machine resumeWithCompletionHandler:handler];
         });
+        Block_release(handler);
     }
 }
 
 void stopWithCompletionHandler(void *machine, void *queue, void *completionHandler)
 {
     if (@available(macOS 12, *)) {
+        handler_t handler = generateHandler(completionHandler);
         dispatch_sync((dispatch_queue_t)queue, ^{
-            [(VZVirtualMachine *)machine stopWithCompletionHandler:^(NSError *err) {
-                virtualMachineCompletionHandler(completionHandler, err);
-            }];
+            [(VZVirtualMachine *)machine stopWithCompletionHandler:handler];
         });
+        Block_release(handler);
     }
 }