Disk image size data are not contained in the reply of query-blockstats
but need to be gathered from query-block. For use with -blockdev we
really need to call 'query-named-block-nodes' and process it to retrieve
the correct data.

This patch introduces qemuMonitorBlockStatsUpdateCapacityBlockdev which
updates the capacity data by nodename rather than device name.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>