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
(cherry picked from commit d46f4a5b6d17f20a9c1de9367af11c9f3e7a7ef0)
Merge "VMware: fix exception when using multiple compute nodes" into stable/havana