qemu: monitor: Retrieve blockstats also by qdev and node-names

qemu: monitor: Retrieve blockstats also by qdev and node-names

For use with -blockdev we need to be able to retrieve the stats by
'qdev' for the frontend device stats since 'device' will be empty. Note
that for non-blockdev case qdev and 'device' with 'drive-' skipped would
be the same.

Additionally so that we can report the highest written offset we need to
also be able to access them by node-name for backing chain purposes.

In cases when 'device' is empty it does not make sense to gather them.

Allow arranging the stats simultaneously in all the above dimensions.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>