block: Accept node-name for blockdev-mirror

block: Accept node-name for blockdev-mirror

In order to remove the necessity to use BlockBackend names in the
external API, we want to allow node-names everywhere. This converts
blockdev-mirror to accept a node-name without lifting the restriction
that we're operating at a root node.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>