Handle vcpu counting failures gracefully

The virDomainGetVcpus() call in libvirt can fail in some cases. This
should be handled properly in nova and not cause failures in starting
up new instances.

Fixes bug 1153694

Change-Id: I8e072c53ee2bafcb50083d793ceabe5292038ab6
Merge "Handle vcpu counting failures gracefully"
Merge remote-tracking branch 'm_src_repo/master' into build-temp