tools: fix check_guests_shutdown loop
    
    The recent fix to libvirt-guests.sh.in works for what it intended to fix
    (variable scope) but failed to adapt the loop in check_guests_shutdown
    correctly. Due to that it currently might detect all guests as "Failed to
    determine state of guest" by bad var content or just assumes they are shut
    down by picking up an empty variable.
    
    This commit fixes loop to use the passed value and the call in the loop
    to actually use the variable assigned in the iterated.
    
    Fixes: 7e476356 "tools: fix variable scope in in check_guests_shutdown"
    Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1764668
    
    Reviewed-by: Dariusz Gadomski <dariusz.gadomski@canonical.com>
    Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>