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>