block: Add drive-mirror-replace command to repair quorum files.

When a quorum file is totally destroyed (broken filer) 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>