fix(test): wait for VM to boot in A/B snapshot test

Otherwise, we might take the snapshot way to early and upon resume
sending the VMGenID notification might crash the guest kernel.

Signed-off-by: Babis Chalios <bchalios@amazon.es>

tests/integration_tests/performance/test_snapshot_ab.py

@@ -145,6 +145,10 @@ def test_restore_latency(
     vm = test_setup.configure_vm(microvm_factory, guest_kernel_linux_4_14, rootfs)
     vm.start()
 
+    # Make sure the guest has booted before taking snapshot.
+    exit_code, _, _ = vm.ssh.run("true")
+    assert exit_code == 0
+
     metrics.set_dimensions(
         {
             "performance_test": "test_restore_latency",