[feature] use async mode in qemu-img

[slavonnet]

Please add -W (async mode) to qemu-img for all posible usage to prevet storage locking

[nirs]

-W is not async, this is unordered writes. This improves performance but not related to locking.

Vdsm already use this option when writing to raw-preallocated disk on block storage. Since vdsm manages all storage operations this issue belongs to vdsm and not engine.

See https://github.com/oVirt/vdsm/blob/242f6f209d6c422ec4f899c33cadfce71b4465ae/lib/vdsm/storage/sd.py#L396

When we added -W, tests with NFS did not show very interesting improvement: https://bugzilla.redhat.com/1511891#c57

In imageio we always use unordered writes, so I think vdsm can use unordered writes for any storage domain or format, but we need to do more testing before we enable this.