Modify two mirror-device related rpc calls

The two calls mirror_addchild and mirror_removechild take only one child
for addition/removal. While this is enough for our md usage, for local
disk replacement in drbd8, we need to be able to specify both the data
and metadata device. This patch modifies these two rpc calls (and their
backend implementation and their usage in cmdlib) to take a list of
children to add/remove.

Reviewed-by: imsnah