getstats: split block stats reporting for easier recursion
    
    In order to report stats on backing chains, we need to separate
    the output of stats for one block from how we traverse blocks.
    
    * src/qemu/qemu_driver.c (qemuDomainGetStatsBlock): Split...
    (qemuDomainGetStatsOneBlock): ...into new helper.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>