qemuMonitorJSONQueryBlockstats: query stats for helper images

qemuMonitorJSONQueryBlockstats: query stats for helper images

Use the 'query-nodes' flag to return all stats. The flag was introduced
prior to qemu-2.11 so we can always use it, but we invoke it only when
querying stats. The other invocation is used for detecting the nodenames
which is fragile code.

The images without a frontend don't have the device field so the
extraction code checks need to be relaxed.

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