vdpa: factor out vdpa_set_features_unlocked for vdpa internal use

vdpa: factor out vdpa_set_features_unlocked for vdpa internal use

No functional change introduced. vdpa bus driver such as virtio_vdpa
or vhost_vdpa is not supposed to take care of the locking for core
by its own. The locked API vdpa_set_features should suffice the
bus driver's need.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/1642206481-30721-2-git-send-email-si-wei.liu@oracle.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>