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.
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