[PATCH 0 of 4] Enable domain checkpointing via xm save --checkpoint

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

Here's take two of xm save --checkpoint. It puts suspend_cancel into a
standalone elfnote. I noticed an off-by-one in my xc.c code to import
elfnotes when I did this; the fix is included here.

>From the last post:

The attached patches create a --checkpoint flag for xm save:
 
xc_domain_resume lets the caller alert the guest that it is being
resumed. Callers should not do this unless they know the guest
supports the operation.
 
When xend wants to resume a domain, it checks for the suspend_cancel
elfnote from the notes it has recorded from domain build or resume. If
the flag is present, it simply lets the guest handle resuming,
otherwise it tears down and rebuilds the domain and lets the guest act
as if it is resuming in a new domain.
 
A new suspend_cancel elfnote is added with value 1.