gui: Watch vm state to terminate when it's stopped

The GUI code in virtualization_view.m is notified when there was a guest
initiated shutdown ('guestDidStopVirtualMachine') and when there was a
virtualization error ('didStopWithError').

When calling Stop(), the VM is forcefully stopped (similar to pulling
the plug on real hardware). This action is neither a guest initiated
shutdown, nor a virtualization error, so the GUI code does not catch
it. This means after calling vm.Stop(), the GUI main loop will keep
running, and the application using Code-Hex/vz will never exit.

This commit fixes this by adding an observer for VM state changes, and
by calling 'terminate' when the VM state becomes 'stopped' or 'error'.

This fixes #150

virtualization_view.m

@@ -165,11 +165,30 @@ - (instancetype)init
 
 @end
 
+API_AVAILABLE(macos(12.0))
+@interface VMStateObserver : NSObject
+- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;
+@end
+
+@implementation VMStateObserver
+- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context;
+{
+    if ([keyPath isEqualToString:@"state"]) {
+        int newState = (int)[change[NSKeyValueChangeNewKey] integerValue];
+        if (newState == VZVirtualMachineStateStopped || newState == VZVirtualMachineStateError) {
+            [NSApp performSelectorOnMainThread:@selector(terminate:) withObject:context waitUntilDone:NO];
+            [object removeObserver:self forKeyPath:@"state"];
+        }
+    }
+}
+@end
+
 @implementation AppDelegate {
     VZVirtualMachine *_virtualMachine;
     VZVirtualMachineView *_virtualMachineView;
     CGFloat _windowWidth;
     CGFloat _windowHeight;
+    VMStateObserver *_observer;
 }
 
 - (instancetype)initWithVirtualMachine:(VZVirtualMachine *)virtualMachine
@@ -179,6 +198,11 @@ - (instancetype)initWithVirtualMachine:(VZVirtualMachine *)virtualMachine
     self = [super init];
     _virtualMachine = virtualMachine;
     [_virtualMachine setDelegate:self];
+    _observer = [[VMStateObserver alloc] init];
+    [virtualMachine addObserver:_observer
+           forKeyPath:@"state"
+              options:NSKeyValueObservingOptionNew
+              context:(void *)self];
 
     // Setup virtual machine view configs
     VZVirtualMachineView *view = [[[VZVirtualMachineView alloc] init] autorelease];