CLOUDSTACK-10227: Add delay before reverting VM snapshot (#2407)

As discovered and discussed in #2376, adding some delay after stopping
the VM and reverting VM snapshot passes the
`test_change_service_offering_for_vm_with_snapshots` test case. The
suspect here is userVMDao or background vmsync that may not update
the VM state to PowerOff.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Author: yadvr

test/integration/smoke/test_vm_snapshots.py

@@ -513,7 +513,8 @@ def test_change_service_offering_for_vm_with_snapshots(self):
             virtual_machine.stop(self.apiclient)
         except Exception as e:
             self.fail("Failed to stop VM: %s" % e)
-        
+
+        time.sleep(30)
         # 8) Revert to VM Snapshot
         self.debug("Revert to vm snapshot: %s" % vm_snapshot.id)
         try: