Run VM state actions in a thread

Prevents blocking the main thread(s) while a VM is changing state. In
particular, this caused some issues with nodes not responding to
cancellation/reversal of a flush/ready state until the previous
migration was finished, which could cause issues. This entire subset of
actions is now threaded and so can run on its own in the background.

node-daemon/pvcd/VMInstance.py

@@ -133,7 +133,9 @@ class VMInstance(object):
                 return False
 
             # Perform a management command
-            self.manage_vm_state()
+            self.logger.out('Updating state of VM {}'.format(self.domuuid), state='i')
+            state_thread = threading.Thread(target=self.manage_vm_state, args=(), kwargs={})
+            state_thread.start()
 
     # Get data functions
     def getstate(self):