It's been marked as deprecated since QEMU v2.10.0, and so far nobody
complained that we should keep it, so let's remove this legacy option
now to simplify the code quite a bit.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>