blkback: make destroy idempotent

We're probably racing with the toolstack, and it's ok to lose the
race.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
blkback: on shutdown report if we missed any requests

If we lost an event it may manifest as a request on the ring we
didn't see. We can check for this just before we unmap.

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