blockjob: add virsh blockcommit

blockjob: add virsh blockcommit

The new command 'virsh blockcommit $dom $disk' requests the start
of an asynchronous commit operation across the entire chain of
$disk.  Further arguments can fine-tune which portion of the
chain is committed.  Existing 'virsh blockjob' commands can then
track the status, change the bandwidth, or abort the commit job.

With a bit more on the command line, 'virsh blockcommit $dom $disk
--wait --verbose' can be used for blocking behavior, with visual
feedback on the overall status, and can be canceled with Ctrl-C.

The overall design, including the wait loop logic, borrows heavily
from the existing blockpull command.

* tools/virsh-domain.c (cmdBlockCommit): New function.
* tools/virsh.pod (blockcommit): Document it.