qcow2: Allow writing compressed data of multiple clusters

qcow2: Allow writing compressed data of multiple clusters

QEMU currently supports writing compressed data of the size equal to
one cluster. This patch allows writing QCOW2 compressed data that
exceed one cluster. Now, we split buffered data into separate clusters
and write them compressed using the block/aio_task API.

Suggested-by: Pavel Butsykin <[email protected]>
Suggested-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Signed-off-by: Andrey Shinkevich <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 1575288906[email protected]
Signed-off-by: Max Reitz <[email protected]>