More simplifications to blkback:

More simplifications to blkback:
 1. Remove blkif->status field as it's really not needed.
 2. Simplify connection logic.
 3. Get rid of atomic_t io_pending. There's no need for
    atomic r-m-w updates to the work-to-do flag, so replace
    with an integer and add barriers where serialisation is
    required.

Signed-off-by: Keir Fraser <keir@xensource.com>