blockcommit: turn on active commit
    
    With this in place, I can (finally!) now do:
    
    virsh blockcommit $dom vda --shallow --verbose --pivot
    
    and watch qemu shorten the backing chain by one, followed by
    libvirt automatically updating the dumpxml output, effectively
    undoing the work of virsh snapshot-commit --no-metadata --disk-only.
    Commit is SOOOO much faster than blockpull, when I'm still fairly
    close in time to when the temporary qcow2 wrapper file was created
    via a snapshot operation!
    
    * src/qemu/qemu_driver.c (qemuDomainBlockCommit): Implement live
    commit.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>