Unimplemented suspend/resume should not change vm state

When the underlying virt driver does not implement the suspend/resume API
and raises a NotImplementedError, the compute manager will error out and
change the instance's vm_state to ERROR. With this change, the compute
manager will handle the NotImplementedError and keep the instance
vm_state as it was before the error.

Change-Id: I8150a898fee91bbe66f1502638baca8050e2dd6d
Fixes: bug #1203986
Merge "Unimplemented suspend/resume should not change vm state"