[libvirt] [PATCH v14] support offline migration

[Wenchao Xia]

   +1 to support adding offline migration API in libvirt, for my
knowledge there were multiple higher management stack in product
did a couple of operations with delete/define to archieve this
goal, but with those fragment operation, higher management need
pay more attention and low level knowledge to handle the exception,
which should be hidden by lower API.
   Pls keep the API unified with online migration, and enhance its
error reporting function as much as possible, so that upper level
stack can report what is wrong to end user.


-- 
Best Regards

Wenchao Xia