qemu: monitor: Add monitor infrastructure for query-named-block-nodes

qemu: monitor: Add monitor infrastructure for query-named-block-nodes

Add monitor tooling for calling query-named-block-nodes. The monitor
returns the data as the raw JSON array that is returned from the
monitor.

Unfortunately the logic to extract the node names for a complete backing
chain will be so complex that I won't be able to extract any meaningful
subset of the data in the monitor code.