This patch introduces access to allocation information about
a backing chain of a live domain.  While querying storage
volumes for read-only disks could provide some of the details,
we do NOT want to read() a file while qemu is writing it.
Also, there is one case where we have to rely on qemu: when
doing a block commit into a backing file, where that file is
stored in qcow2 format on a host block device, we want to know
the current highest write offset into that image, in order to
know if the disk must be resized larger.  qemu-img does not
(currently) show this information, and none of the earlier
block APIs were extensible enough to expose it.  But
virDomainListGetStats is perfect for the job!

We don't need a new group of statistics, as the existing block
group is sufficient.  On the other hand, as existing libvirt
releases already report 1:1 mapping of block.count to <disk>
devices, changing the array size could confuse older clients;
and even with newer clients, the time and memory taken to
report additional statistics is not always necessary (backing
files are generally read-only except for block-commit, so while
read statistics may change, sizing statistics will not).  So
the choice here is to add a new flag that only newer callers
will pass, when they are prepared for the additional information.

This patch introduces the new API, but it will take more
patches to get it implemented for qemu.

* include/libvirt/libvirt-domain.h
(VIR_CONNECT_GET_ALL_DOMAINS_STATS_BACKING): New flag.
* src/libvirt-domain.c (virConnectGetAllDomainStats): Document it,
and add a new field when it is in use.
* tools/virsh-domain-monitor.c (cmdDomstats): Use new flag.
* tools/virsh.pod (domstats): Document it.
Signed-off-by: NEric Blake <eblake@redhat.com>