block: Relax bdrv_lookup_bs constraints.

The following patch will reuse bdrv_lookup_bs in order to open images by
references so the rules of usage of bdrv_lookup_bs must be relaxed a bit.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
block: Open by reference will try device then node_name.

Since we introduced node_name for named bs of the graph modify the opening by
reference to use it as a fallback.

This patch also enforce the separation of the device id and graph node
namespaces.

Signed-off-by: Benoit Canet <benoit@irqsave.net>