While libvirt-guests.sh is running cases can let guest_is_on fail which
causes check_guests_shutdown to print output.
That output shall not spill into the users of function
check_guests_shutdown which is therefore now returning values in a
variable like guest_is_on already did.
Original-Author: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Modified-By: Jorge Niedbalski <niedbalski@ubuntu.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>