libvirt: Unnecessary instance.save(s) called

When spawning an instance there may a considerable amount of
instance.save() called. This is not necessary as the compute
manager will update the instance after the spawn method has
successfully completed.

Change-Id: I61610b3869caeb7822eab3f2d09335744b75db28
Closes-bug: #1366548
Merge "libvirt: Unnecessary instance.save(s) called"