xen/pvh: Implement MMU changes for PVH.

xen/pvh: Implement MMU changes for PVH.

.. which are surprinsingly small compared to the amount for PV.
First the set/clear mmio pte function make a hypercall to update the
P2M in Xen with 1:1 mapping. Since PVH uses mostly native mmu ops, we
leave the generic (native_*) for the majority and just overwrite the
baremetal with the ones we need.

Two local functions are introduced to add to Xen physmap for Xen remap
interface. Xen unmap interface is introduced so that the privcmd PTe entries
can be cleared in Xen P2M table.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>