Clean up shadow-translate xen patches. Add abstractions

Clean up shadow-translate xen patches. Add abstractions
for adding/removing pages from a translated guest's
physmap. Define dummy functions so other architectures
will continue to build without errors.

Remove setting of XENFEAT_writable_mmu_structures. This
should set only if the hypervisor supports writable
mappings of all MMU structures (all page tables and
descriptor tables). If we want a mode where only PTEs
can be writable, we should add a feature flag for that
(but I don't think this is a useful mode to support).

TODO: The call to get the pfn hole should be a
XENMEM_ function, not another MMUEXT_OP (hopefully the
latter hypercall is not goign to grow any more as it's
gross enough already).

Signed-off-by: Keir Fraser <keir@xensource.com>