Rename it to qemuBlockStorageSourceGetBackendProps and refactor it to
return the JSON object instead of filling a pointer since now it's
always expected to return data.
To allow updating stats based on the node name, add a helper function
that will fetch the required data from 'query-named-block-nodes' and
return it in hash table for easy lookup.
Detect the node names when setting block threshold and when reconnecting
or when they are cleared when a block job finishes. This operation will
become a no-op once we fully support node names.
qemu for some time already sets node names automatically for the block
nodes. This patch adds code that attempts a best-effort detection of the
node names for the backing chain from the output of
'query-named-block-nodes'. The only drawback is that the data provided
by qemu needs to be matched by the filename as seen by qemu and thus
if two disks share a single backing store file the detection won't work.
This will allow us to use qemu commands such as
'block-set-write-threshold' which only accepts node names.
In this patch only the detection code is added, it will be used later.
