We need to do this so we can have a migration uuid at the time we
call to scheduler to allocate for the new host. This just does the
plumbing through the RPC layers. The compute-side code can already
tolerate a migration having been already created for things like
live migration, so we just have to plumb it through.
Related to blueprint migration-allocations
Change-Id: I6bc6d28655368084f08fed9c4f56a285b7063338