virtio-scsi: convert virtio_scsi_bad_req() to use virtio_error()

The virtio_scsi_bad_req() function is called when a guest sends a
request with missing or ill-sized headers. This generally happens
when the virtio_scsi_parse_req() function returns an error.

With this patch, virtio_scsi_bad_req() will mark the device as broken,
detach the request from the virtqueue and free it, instead of forcing
QEMU to exit.

In nearly all locations where virtio_scsi_bad_req() is called, the only
thing to do next is to return to the caller.

The virtio_scsi_handle_cmd_req_prepare() function is an exception though.

It is called in a loop by virtio_scsi_handle_cmd_vq() and passed requests
freshly popped from a cmd virtqueue; virtio_scsi_handle_cmd_req_prepare()
does some sanity checks on the request and returns a boolean flag to
indicate whether the request should be queued or not. In the latter case,
virtio_scsi_handle_cmd_req_prepare() has detected a non-fatal error and
sent a response back to the guest.

We have now a new condition to take into account: the device is broken
and should stop all processing.

The return value of virtio_scsi_handle_cmd_req_prepare() is hence changed
to an int. A return value of zero means that the request should be queued.
Other non-fatal error cases where the request shoudn't be queued  return
a negative errno (values are vaguely inspired by the error condition, but
the only goal here is to discriminate the case we're interested in).

And finally, if virtio_scsi_bad_req() was called, -EINVAL is returned. In
this case, virtio_scsi_handle_cmd_vq() detaches and frees already queued
requests, instead of submitting them.

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>
