vdpa: Move vhost_svq_poll() to the caller of vhost_vdpa_net_cvq_add()

vdpa: Move vhost_svq_poll() to the caller of vhost_vdpa_net_cvq_add()

This patch moves vhost_svq_poll() to the caller of
vhost_vdpa_net_cvq_add() and introduces a helper funtion.

By making this change, next patches in this series is
able to refactor vhost_vdpa_net_load_x() only to delay
the polling and checking process until either the SVQ
is full or control commands shadow buffers are full.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <196cadb55175a75275660c6634a538289f027ae3.1697165821.git.yin31149@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>