xl: New savefile format. Save domain config when saving a domain.

xl: New savefile format. Save domain config when saving a domain.

We introduce a new format for saved domains.  The new format, in
contrast to the old:
  * Has a magic number which can distinguish it from other kinds of
  file
  * Is extensible
  * Can contains the domain configuration file

On domain creation we remember the actual config file used (using the
toolstack data feature of libxl, just introduced), and by default save
it to the save file.

However, options are provided for the following:
  * When saving a domain, supplying an alternative config file to
    store in the savefile.
  * When restoring a domain, supplying an alternative config file.

If a domain is restored with a different config file, it is the
responsibility of the xl user to ensure that the two configs are
"compatible".  Changing the targets of virtual devices is supported;
changing other features of the domain is not recommended.  Bad changes
may lead to undefined behaviour in the domain, and are in practice
likely to cause resume failures or crashes.

Old format save files generated by old versions of xl are not
supported.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>