iotests: add test for changing mirror's copy_mode

iotests: add test for changing mirror's copy_mode

One part of the test is using a throttled source to ensure that there
are no obvious issues when changing the copy_mode while there are
ongoing requests (source and target images are compared at the very
end).

The other part of the test is using a throttled target to ensure that
the change to active mode actually happened. This is done by hitting
the throttling limit, issuing a synchronous write and then immediately
verifying the target side. QSD is used, because otherwise, a
synchronous write would hang there.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20231031135431.393137-11-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>