quorum: Add the rewrite-corrupted parameter to quorum.

On read operations when this parameter is set and some replicas are corrupted
while quorum can be reached quorum will proceed to rewrite the correct version
of the data to fix the corrupted replicas.

This will shine with SSD where the FTL will remap the same block at another
place on rewrite.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Max Reitz <mreitz@redhat.com>
block: Add drive-mirror-replace command to repair quorum files.

When a quorum file is totally destroyed (broken NAS or SAN) the user can start a
drive-mirror job on the quorum block backend and then replace the broken
quorum file with drive-mirror-replace given it has a node-name.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
qemu-iotests: Add 088 new test for drive-mirror-replace.

Tests for drive-mirror-replace whose purpose is to enable quorum file mirroring
and replacement after failure.

Signed-off-by: Benoit Canet <benoit@irqsave.net>