9pfs: fix crash when fsdev is missing

If the user passes -device virtio-9p without the corresponding -fsdev, QEMU
dereferences a NULL pointer and crashes.

This is a 2.8 regression introduced by commit 702dbcc274e2c.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
