blkfront: update following Gnttab -> Gnt API change

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
We don't need to conditionally compile blkback and blkfront

Since the libraries have been unified, we should be able to
build both.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.1.2

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Regenerate OASIS

Signed-off-by: David Scott <dave.scott@eu.citrix.com>