hw/virtio: move vhost_set_backend_type() to vhost.c

hw/virtio: move vhost_set_backend_type() to vhost.c

Just a small refactor patch.

vhost_set_backend_type() gets called only in vhost.c, so we can move the
function there and make it static. We can then extern the visibility of
kernel_ops, to match the other VhostOps in vhost-backend.h.
The VhostOps constants now make more sense in vhost.h

Suggested-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@nutanix.com>
Message-Id: <20210809134015.67941-1-tiberiu.georgescu@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>