blockdev: enable non-root nodes for backup source

This is needed to implement the image-fleecing workflow where we
create a temporary node backed by an active node, then start
backupdev-backup sync=none from the active node to the temp node.

In this case, the active node is now a root node AND a backing node,
so it no longer qualifies as a root node, so we loosen the restriction
on which nodes can be considered as the source for a backup.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20180702194630.9360-2-jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>