vhost: Get vring base from vq, not svq

vhost: Get vring base from vq, not svq

The SVQ vring used idx usually match with the guest visible one, as long
as all the guest buffers (GPA) maps to exactly one buffer within qemu's
VA. However, as we can see in virtqueue_map_desc, a single guest buffer
could map to many buffers in SVQ vring.

Also, its also a mistake to rewind them at the source of migration.
Since VirtQueue is able to migrate the inflight descriptors, its
responsability of the destination to perform the rewind just in case it
cannot report the inflight descriptors to the device.

This makes easier to migrate between backends or to recover them in
vhost devices that support set in flight descriptors.

Fixes: 6d0b22266633 ("vdpa: Adapt vhost_vdpa_get_vring_base to SVQ")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>