conf: Add 'index' attribute for <disk><mirror><source>

conf: Add 'index' attribute for <disk><mirror><source>

Similarly to the disk source we need to keep the disk index (which is in
the qemu driver used for identification of the source for block jobs)
for the <mirror> element so that when it's replaced as a disk source
after pivoting all the allocated data is present.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>