x86/HVM: move vendor independent CPU save/restore logic to shared code

x86/HVM: move vendor independent CPU save/restore logic to shared code

A few pieces of the handling here are (no longer?) vendor specific, and
hence there's no point in replicating the code. Zero the full structure
before calling the save hook, eliminating the need for the hook
functions to zero individual fields.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>