virtio-9p: handle handle_9p_output() error

A broken guest may send a request without providing buffers for the reply
or for the request itself, and virtqueue_pop() will return an element with
either in_num == 0 or out_num == 0.

All 9P requests are expected to start with the following 7-byte header:

            uint32_t size_le;
            uint8_t id;
            uint16_t tag_le;

If iov_to_buf() fails to return these 7 bytes, then something is wrong in
the guest.

In both cases, it is wrong to crash QEMU, since the root cause lies in the
guest.

This patch hence does the following:
- keep the check of in_num since pdu_complete() assumes it has enough
  space to store the reply and we will send something broken to the guest
- let iov_to_buf() handle out_num == 0, since it will return 0 just like
  if the guest had provided an zero-sized buffer.
- call virtio_error() to inform the guest that the device is now broken,
  instead of aborting
- detach the request from the virtqueue and free it

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
