x86: move pv-only members of struct vcpu to struct pv_vcpu

x86: move pv-only members of struct vcpu to struct pv_vcpu

... thus further shrinking overall size of struct arch_vcpu.

This has a minor effect on XEN_DOMCTL_{get,set}_ext_vcpucontext - for
HVM guests, some meaningless fields will no longer get stored or
retrieved: reads will now return zero, and writes are required to be
(mostly) zero (the same as was already done on x86-32).

Signed-off-by: Jan Beulich <jbeulich@novell.com>