block/iscsi: use the allocationmap also if cache.direct=on

the allocationmap has only a hint character. The driver always
double checks that blocks marked unallocated in the cache are
still unallocated before taking the fast path and return zeroes.
So using the allocationmap is migration safe and can
also be enabled with cache.direct=on.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1429193313-4263-10-git-send-email-pl@kamp.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
