tools/libxc+libxl+xl: Save v2 streams

tools/libxc+libxl+xl: Save 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 save operations.
 * libxl__stream_write_start() is logically spliced into the event location
   where libxl__xc_domain_save() used to reside.
 * Ownership of the helper state moves into stream_write_state.
 * xl is updated to indicate that the stream is now v2

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: Many small changes, follwing similar review to the restore side