Renable sleep at Vbox exit bug seem to be back

Fix https://github.com/GNS3/gns3-gui/issues/1444

gns3server/modules/virtualbox/virtualbox_vm.py

@@ -250,7 +250,7 @@ class VirtualBoxVM(BaseVM):
                 log.debug("Stop result: {}".format(result))
 
             log.info("VirtualBox VM '{name}' [{id}] stopped".format(name=self.name, id=self.id))
-            # yield from asyncio.sleep(0.5)  # give some time for VirtualBox to unlock the VM
+            yield from asyncio.sleep(0.5)  # give some time for VirtualBox to unlock the VM
             try:
                 # deactivate the first serial port
                 yield from self._modify_vm("--uart1 off")