vdpa: Use iovec for vhost_vdpa_net_cvq_add()

vdpa: Use iovec for vhost_vdpa_net_cvq_add()

Next patches in this series will no longer perform an
immediate poll and check of the device's used buffers
for each CVQ state load command. Consequently, there
will be multiple pending buffers in the shadow VirtQueue,
making it a must for every control command to have its
own buffer.

To achieve this, this patch refactor vhost_vdpa_net_cvq_add()
to accept `struct iovec`, which eliminates the coupling of
control commands to `s->cvq_cmd_out_buffer` and `s->status`,
allowing them to use their own buffer.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <8a328f146fb043f34edb75ba6d043d2d6de88f99.1697165821.git.yin31149@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>