This patch adds the final missing bits for support of
passing a serial/id string to a virtio-blk guest driver.

The guest-side component already exists in the virtio
driver, and has recently been reworked by Ryan to export
a /sys interface for retrieval of the id from guest userland.
Signed-off-by: Njohn cooper <john.cooper@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>