tools/libxc+libxl+xl: Restore v2 streams

tools/libxc+libxl+xl: Restore v2 streams

This is a complicated set of changes which must be done together for
bisectability.

 * libxl-save-helper is updated to unconditionally use libxc migration
   v2.
 * libxl compatibility workarounds in libxc are disabled for restore
   operations.
 * libxl__stream_read_start() is logically spliced into the event
   location where libxl__xc_domain_restore() used to reside.
 * Ownership of the save_helper_state moves to stream_read_state.

The parameters 'hvm', 'pae', and 'superpages' were previously
superfluous, and are completely unused in migration
v2. callbacks->toolstack_restore is handled via a migration v2 record
now, rather than via a callback from libxc.

NB: this change breaks Remus.  Further untangling needs to happen
before Remus will function.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
---
v4:
 * Don't use _init() needlessly
v3:
 * Simplify from v2.
 * Alter the ownership of save_helper_state

v2:
 * Drop "legacy_width" from the IDL
 * Gain a LIBXL_HAVE_ to signify support of migration v2 streams