vdpa: use v->shadow_vqs_enabled in vhost_vdpa_svqs_start & stop

vdpa: use v->shadow_vqs_enabled in vhost_vdpa_svqs_start & stop

This function used to trust in v->shadow_vqs != NULL to know if it must
start svq or not.

This is not going to be valid anymore, as qemu is going to allocate svq
array unconditionally (but it will only start them conditionally).

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20221215113144.322011-2-eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>