blkback: update following Gnttab -> Gnt.gnttab API change

Note also that Activations.wake was being called, I believe, as a
workaround for a previous 'ack_requests' function which only checked
for requests once. The current 'ack_requests' function will loop
forever while there is work to do.

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