tools/libxl: Add save_helper_state pointers to libxl__xc_domain_{save,restore}()

tools/libxl: Add save_helper_state pointers to libxl__xc_domain_{save,restore}()

Currently, libxl__xc_domain_{save,restore}() have specific knowledge
of where the libxl__save_helper_state lives inside a
libxl__domain_{create,save}_state object.

In later changes, the logical ownership of the
libxl__save_helper_state will change and will no longer be
d{c,s}s->shs.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
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New in v3