libvirt: speed up get_vcpus method

Switch the get_vcpus method method to use the new
list_instance_domains method.

Blueprint: libvirt-domain-listing-speedup
Change-Id: I474d6db502d19f2296252f4f3ae2ef740a2188e8
Remove cell api overrides for lock and unlock

This isn't actually needed as these methods handle objects and is
triggered when the instance object is saved.
It actually caused a race condition with unlock as it triggers 2
instance updates from a compute cell.

Change-Id: I42eea6a2984f70527d4fc37f48ad973a2248721c
Closes-Bug: #1257168
Merge "Remove cell api overrides for lock and unlock"
Merge "libvirt: speed up get_vcpus method"