xen/pvh: Create a new file for Xen specific PVH code

We need to refactor PVH entry code so that support for other hypervisors
like Qemu/KVM can be added more easily.

The first step in that direction is to create a new file that will
eventually hold the Xen specific routines.

Signed-off-by: Maran Wilson <maran.wilson@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>