qemu-img: make convert async

qemu-img: make convert async

the convert process is currently completely implemented with sync operations.
That means it reads one buffer and then writes it. No parallelism and each sync
request takes as long as it takes until it is completed.

This can be a big performance hit when the convert process reads and writes
to devices which do not benefit from kernel readahead or pagecache.
In our environment we heavily have the following two use cases when using
qemu-img convert.

a) reading from NFS and writing to iSCSI for deploying templates
b) reading from iSCSI and writing to NFS for backups

In both processes we use libiscsi and libnfs so we have no kernel cache.

This patch changes the convert process to work with parallel running coroutines
which can significantly improve performance for network storage devices:

qemu-img (master)
 nfs -> iscsi 22.8 secs
 nfs -> ram   11.7 secs
 ram -> iscsi 12.3 secs

qemu-img-async (8 coroutines, in-order write disabled)
 nfs -> iscsi 11.0 secs
 nfs -> ram   10.4 secs
 ram -> iscsi  9.0 secs

This patches introduces 2 new cmdline parameters. The -m parameter to specify
the number of coroutines running in parallel (defaults to 8). And the -W parameter to
allow qemu-img to write to the target out of order rather than sequential. This improves
performance as the writes do not have to wait for each other to complete.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>