python/aqmp: Fix negotiation with pre-"oob" QEMU

QEMU versions prior to the "oob" capability *also* can't accept the
"enable" keyword argument at all. Fix the handshake process with older
QEMU versions.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20220201041134.1237016-2-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>