Creating an instance of qemu.aqmp.ExecuteError is too involved here, so
just drop the specificity down to a generic QMPError.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>