getstats: report block sizes for offline domains
    
    The prior refactoring can now be put to use. With the same domain
    as the earlier commit 7b49926 (one qcow2 disk and an empty
    cdrom drive):
    $ virsh domstats --block foo
    Domain: 'foo'
      block.count=2
      block.0.name=hda
      block.0.path=/var/lib/libvirt/images/foo.qcow2
      block.0.allocation=1309614080
      block.0.capacity=42949672960
      block.0.physical=1309671424
      block.1.name=hdc
    
    * src/qemu/qemu_driver.c (qemuDomainGetStatsBlock): Use
    qemuStorageLimitsRefresh to report offline statistics.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>