util: more verbose error when failing to create macvtap device
    
    Investigation of a problem with creating passthrough macvtap devices
    (https://bugzilla.redhat.com/show_bug.cgi?id=1185501) has shown that
    this slightly more verbose failure message is useful. In particular,
    the mac address can be used to determine the domain. You could also
    figure this out by looking at preceding messages in a debug log, but
    this gets it in a single place.