getstats: prepare for dynamic block.count stat

getstats: prepare for dynamic block.count stat

A coming patch will make it optionally possible to list backing
chain block stats; in this mode of operation, block.counts is no
longer the number of <disks> in the domain, but the number of
blocks in the array being reported.  We still want block.count
listed first, but rather than iterate the tree twice (once to
count, and once to list stats), it's easier to just touch things
up after the fact.

* src/qemu/qemu_driver.c (qemuDomainGetStatsBlock): Compute count
after the fact.

Signed-off-by: Eric Blake <eblake@redhat.com>