Format the address width attribute. Depending on the version of
QEMU it is named 'aw-bits' or 'x-aw-bits'.
Signed-off-by: Menno Lageman <menno.lageman@oracle.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>