tools: avoid text spilling into variables
    
    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>