libvirt pmda: adjust block dev metrics collection

libvirt pmda: adjust block dev metrics collection

libvirt.git/src/libvirt-domain.c says:

 * VIR_DOMAIN_STATS_BLOCK: Return block devices statistics.  By default,
 * this information is limited to the active layer of each <disk> of the
 * domain (where block.count is equal to the number of disks), but adding
 * VIR_CONNECT_GET_ALL_DOMAINS_STATS_BACKING to @flags will expand the
 * array to cover backing chains (block.count corresponds to the number
 * of host resources used together to provide the guest disks).

I think we could perhaps at least start without that flag, and if needed
then provide an configuration option to enable it.  For reference, in case
the backing flag is considered later, this comment from the file mentioned
above should be kept in mind:
 * "block.<num>.backingIndex" - unsigned int giving the <backingStore> index,
 *                              only used when backing images are listed.