VMware: prevent race condition with VNC port allocation

When spawning some instances, nova VMware driver could have a race condition
in VNC port allocation. This fix ensures that the lock is done on the
actual setting in the VM configuration spec.

Co-authored-by: Marcio Roberto Starke <marcio.starke@serpro.gov.br>

Change-Id: I70fab021bbf2df418df53e5f47e19cf16dbe45ac
Closes-bug: #1357372
(cherry picked from commit 948ff4f3d0a159f1aed9fab65d205ede845b3eb9)
Merge "VMware: prevent race condition with VNC port allocation" into stable/icehouse