Fix Live-Migrate problems

The patches that follow address a series of problems that we have found when migrating PV-on-HVM guests against unstable.

In order to get migration to work with legacy guests, we also needed to apply the following changesets from the staging tree:
changeset 15800: fba9884685fb <http://xenbits.xensource.com/staging/xen-unstable.hg?rev/fba9884685fb>
changeset 15799: 3738840029b4 <http://xenbits.xensource.com/staging/xen-unstable.hg?rev/3738840029b4>

Thanks for these. I think 1/3 and 3/3 can be fixed more neatly, but the main work here was obviously tracking down the bugs rather than concocting the patches. I'll check in suitable equivalent fixes tomorrow.

1/3: We can just use kernel_thread()+set_cpus_allowed() rather than the kthread mechanisms.
3/3: Better to promote the offending inline function into a .c file.

-- Keir