qemu: Do not reset labels when migration fails
    
    When stopping a domain on the destination host after a failed migration,
    we need to avoid reseting security labels since the domain is still
    running on the source host. While we were correctly doing so in some
    cases, there were still some paths which did this wrong.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1242904
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>