[libvirt] Adding migration speed to domainXML

[Jim Fehlig]

I finally have some time to investigate Eric's suggestion [1] to add
virDomainMigrateGetMaxSpeed API and track current value in domainXML. 
WRT the XML representation, it seems this best fits under the existing
lifecycle controls, e.g. a new "on_migrate" element.  I'm not sure if
this should be kept simple and rather flat such as

<on_migrate max-speed='1000000000' />

or a little more expressive and extensible with something like

<on_migrate>
  <bandwidth peak='1000000000' />
</on_migrate>

Any comments on these representations or perhaps alternative suggestions?

Regards,
Jim

[1] https://www.redhat.com/archives/libvir-list/2011-August/msg00224.html