vdpa: Introduce cursors to vhost_vdpa_net_loadx()

This patch introduces two new arugments, `out_cursor`
and `in_cursor`, to vhost_vdpa_net_loadx(). Addtionally,
it includes a helper function
vhost_vdpa_net_load_cursor_reset() for resetting these
cursors.

Furthermore, this patch refactors vhost_vdpa_net_load_cmd()
so that vhost_vdpa_net_load_cmd() prepares buffers
for the device using the cursors arguments, instead
of directly accesses `s->cvq_cmd_out_buffer` and
`s->status` fields.

By making these change, next patches in this series
can refactor vhost_vdpa_net_load_cmd() directly to
iterate through the control commands shadow buffers,
allowing QEMU to send CVQ state load commands in parallel
at device startup.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <1c6516e233a14cc222f0884e148e4e1adceda78d.1697165821.git.yin31149@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>