libxl: domain restore: reshuffle, preparing for ao

libxl: domain restore: reshuffle, preparing for ao

We are going to arrange that libxl, instead of calling
xc_domain_restore, calls a stub function which forks and execs a
helper program, so that restore can be asynchronous rather than
blocking the whole toolstack.

This stub function will be called libxl__xc_domain_restore.

However, its prospective call site is unsuitable for a function which
needs to make a callback, and is buried in two nested single-call-site
functions which are logically part of the domain creation procedure.

So we first abolish those single-call-site functions, integrate their
contents into domain creation in their proper temporal order, and
break out libxl__xc_domain_restore ready for its reimplementation.

No functional change - just the following reorganisation:

* Abolish libxl__domain_restore_common, as it had only one caller.
  Move its contents into (what was) domain_restore.

* There is a new stage function domcreate_rebuild_done containing what
  used to be the bulk of domcreate_bootloader_done, since
  domcreate_bootloader_done now simply starts the restore (or does the
  rebuild) and arranges to call the next stage.

* Move the contents of domain_restore into its correct place in the
  domain creation sequence.  We put it inside
  domcreate_bootloader_done, which now either calls
  libxl__xc_domain_restore which will call the new function
  domcreate_rebuild_done, or calls domcreate_rebuild_done directly.

* Various general-purpose local variables (`i' etc.) and convenience
  alias variables need to be shuffled about accordingly.

* Consequently libxl__toolstack_restore needs to gain external linkage
  as it is now in a different file to its user.

* Move the xc_domain_save callbacks struct from the stack into
  libxl__domain_create_state.

In general the moved code remains almost identical.  Two returns in
what used to be libxl__domain_restore_common have been changed to set
the return value and "goto out", and the call sites for the abolished
and new functions have been adjusted.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>