Service monitor changes for VM services

- Store VMI information since delete by Nova and
  API server are synced. Hence a new VM needs to
  be launched with a different fully qualified name.

- Add virtual machine state information to DB so
  that status check is not started until SI is active.

- Add exception handling for Nova calls and return None
  for cases of exception to the clients.

Change-Id: I4037547ce81c1712697115b49a3d4eaa1002c8af
Closes-Bug: #1390396
Merge "Service monitor changes for VM services"