xen64: xen_write_idt_entry() and cvt_gate_to_trap()

Changed to use the (to-be-)unified descriptor structs.

Signed-off-by: Eduardo Habkost <ehabkost@Rawhide-64.localdomain>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>