block/qcow2: introduce parallel subrequest handling in read and write

It improves performance for fragmented qcow2 images.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190916175324.18478-6-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
3 files changed