The way to specify the node from which to copy data in the
block-stream operation is by using the 'base' parameter. This
parameter however takes a file name, not a node name.

Since we want to be able to perform this operation using only node
names, this patch adds a new 'base-node' parameter.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>