blktap: Segments can span multiple clusters with tap:qcow.

blktap: Segments can span multiple clusters with tap:qcow.

In blktap's qcow we need split up read/write requests if the requests
span multiple clusters. However, with our MAX_AIO_REQUESTS define we
assume that there is only ever a single aio request per tapdisk
request and under heavy i/o we can run out of room causing us to
cancel requests.

Dynamically allocate (based on cluster_bits) the various io request
queues the driver maintains.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>