V2 Pause: treat case when driver does not implement the operation

Some virt drivers may not support this operation.

Same change will be added to V3 when https://review.openstack.org/#/c/58432
is merged.

Change-Id: I7763f462dd8f61c29c79038d881dc015482a9b5c
Merge "V2 Pause: treat case when driver does not implement the operation"