xen: defer building p2m mfn structures until kernel is mapped

When building mfn parts of p2m structure, we rely on being able to
use mfn_to_virt, which in turn requires kernel to be mapped into
the linear area (which is distinct from the kernel image mapping
on 64-bit).  Defer calling xen_build_mfn_list_list() until after
xen_setup_kernel_pagetable();

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>