ivshmem: fix ivshmem-{plain,doorbell} crash without arg

"qemu -device ivshmem-{plain,doorbell}" will crash, because the device
doesn't check that the required argument is provided. (screwed up in
commit 5400c02)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
