qemu: Check for user alias collisions in coldplug
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1697676
    
    If an user tries to attach a device with colliding user alias
    then we attach it happily and thus leave domain unable to start.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>