libvirt: return after soft reboot successfully completes
libvirt: return after soft reboot successfully completes

A soft reboot shuts down an instance, waits for the instance to shutdown,
then starts the instance and waits for the instance to finish starting.
However, instead of returning success, it continues back into the loop
and waits for the instance to be shutdown again.

That will continue until the soft reboot times out, which is by default
120 seconds.

This manifested itself as a unit test that would consistently take 120
seconds to run, but could possibly affect deployed environments by
restarting instances that are shutdown shortly after a soft reboot.

Change-Id: I012c53343be4b4eb943643903fc47c7fe70a594d

NewpTone/stacklab-nova,NewpTone/stacklab-nova,paulmathews/nova,paulmathews/nova,paulmathews/nova,savi-dev/nova,savi-dev/nova,savi-dev/nova,NewpTone/stacklab-nova