CP-8987: Use Sexp for Xenops header serialisation

We want to be able to extend the Xenops record in the suspend image in future
versions. We therefore require a serialisation format that supports optional
parsing. Sexplib has such functionality with sexp_option allowing us to add new
fields to new versions and still parse older versions with these defaulting to
None.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-8987: Add domain's Xenstore tree to the Xenops suspend record

Add everything under /local/domain/<domid>/ to the suspend image as part of the
Xenops metadata record.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-8987: Serialise Xenops VM record into suspend image

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-8987: Don't log Xenops suspend image record contents

It is too large to log when reading and writing but will be of use when
inspecing a suspend image.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #87 from simonjbeaumont/migv2-metadata

CP-8987: Support extensible Xenops suspend image records