Fix HVM direct kernel booting.

Fix HVM direct kernel booting.

This was broken during the merge with qemu upstream.

Fixes:
 * Properly update kernel loading address so that qemu actually
   loads the kernel where we want it.
 * Use 0x200000 not 0x20000 as the updated kernel loading address.
 * Use stl_phys(real_addr+0x214,) rather than stl_p(header+0x214,),
   as the header has already been copied into target memory so we
   need to update it there.