virtio-blk: make some functions static

virtio-blk: make some functions static

Some functions that were called from the dataplane code are now only used
locally:

virtio_blk_init_request()
virtio_blk_handle_request()
virtio_blk_submit_multireq()

since commit "03de2f527499 virtio-blk: do not use vring in dataplane", and

virtio_blk_free_request()

since commit "6aa46d8ff1ee virtio: move VirtQueueElement at the beginning
of the structs".

This patch converts them to static.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>