blockcommit: turn on active commit

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>