[PATCH 0 of 7] guest checkpointing patches take 3

[Brendan Cully <brendan@cs.ubc.ca>]

Here's version 3 of the checkpointing code. It retains backward
compatibility with older kernels and tools by setting the return code
of the suspend hypercall to 1 in the parent domain (a case never seen
by older versions) and leaving it set to 0 in the child. It also adds
an untested 'unsuspend' hook that devices can use to roll back any
suspend prep work they may have done.

It still advertises checkpointability via a feature flag in
xenstore. I'm putting the patches out now because the elfnote approach
is a) mainly a cosmetic difference and b) likely to take a few rounds
of review for itself. In the meantime, I think these patches are
suitable for testing.

Thanks,
Brendan