xen: add PVHv2 entry point

The PVHv2 entry point is fairly similar to the multiboot1 one. The
kernel is started in protected mode with paging disabled. More
information about the exact BSP state can be found in the pvh.markdown
document on the Xen tree.

This entry point is going to be joined with the native entry point at
hammer_time, and in order to do so the BSP needs to be bootstrapped
into long mode with the same set of page tables as used on bare metal.

Sponsored by: Citrix Systems R&D