xen/mmu: Support P->M array to be in the VMEMMAP_START virtual address.

This is done to support booting with more than 256GB for initial
domain.

TODO: Actually make it work :-)

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

Conflicts:

	arch/x86/xen/mmu.c
6 files changed