vl: Deprecate -virtfs_synth

The synth fsdriver never got used for anything else but the QTest
testcase for VirtIO 9P. And even there, QTest uses -fsdev synth and
-device virtio-9p-... directly.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>