block: Improve error message when inserting a duplicate node-name.

Let the user know which node-name is concerned.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
block: Add node-name and to-replace-node-name arguments to drive-mirror.

node-name give a name to the created BDS and register it in the node graph.

to-replace-node-name can be used when drive-mirror is called with sync=full.

The purpose of these fields is to be able to reconstruct and replace a broken
quorum file.

drive-mirror will bdrv_swap the new BDS named node-name with the one
pointed by to-replace-node-name when the mirroring is finished.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
qemu-iotests: Add TestRepairQuorum to 041 to test drive-mirror node-name mode.

The the to-replace-node-name is designed to allow repairing of broken Quorum
file.
This patch introduce a new class TestRepairQuorum testing that the feature
works.
Some further work will be done on QEMU to improve the robutness of the tests.

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