VMware: fix exception when using multiple compute nodes

When there is more than one compute node running and one of the
nodes restarts it terminates on an exception that the resource is
not found.

The cause of the issue is that a vif plug was being attempted for
a resource that did not exist. The vif plug should have raised an
"NotImplemented" exception.

Change-Id: I5a3f1cc73a981173b6c2fa493de3aad10a7e97fd
Closes-bug: #1272286
Merge "VMware: fix exception when using multiple compute nodes"