xen: use the same hypercall mechanism for XEN and XENHVM

Currently XEN (PV) and XENHVM (PVHVM) ports use different ways to
issue hypercalls, unify this by filling the hypercall_page under HVM
also.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

amd64/include/xen/hypercall.h:

• Unify Xen hypercall code by always using the PV way.

i386/i386/locore.s:

• Define hypercall_page on i386 XENHVM.

x86/xen/hvm.c:

• Fill hypercall_page on XENHVM kernels using the HVM method (only when running as an HVM guest).