libxl: Rewind toolstack_save_fd in libxl_save_helper when using remus

libxl: Rewind toolstack_save_fd in libxl_save_helper when using remus

Commit b327a3f421bb57d262b7d1fb3c43b710852b103b moved the rewinding of
toolstack_save_fd to libxl.  This breaks remus, because in remus mode,
toolstack_save_cb will be called in every checkpoint, and if we don't
rewind it in libxl_save_helper, it will surely fail.

This fix is just a hack: in fact the whole toolstack save thing should
be done in libxl.  But for now (until migration v2) this fix should
solve both remus and Jason Adryuk's use case.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Tested-by: Jason Andryuk <andryuk@aero.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>