x86/svm: Drop the set_segment_register() macro

x86/svm: Drop the set_segment_register() macro

Replace its sole users with a single piece of inline assembly which is more
flexable about its register constraints, rather than forcing the use of %ax.

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No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>