x86/shadow: move shadow_set_l<N>e() to their own source file

The few GUEST_PAGING_LEVELS dependencies (of shadow_set_l2e() only) can
be easily expressed by function parameters; I suppose the extra indirect
call is acceptable for the increasingly little used 32-bit non-PAE case.
This way shadow_set_l[12]e(), each of which compiles to almost 1k of
code, need building just once.

The implication is the need for some "relaxation" in types.h: The
underlying PTE types don't vary anymore (and aren't expected to down the
road), so they as well as some basic helpers can be exposed even in the
new, artificial GUEST_PAGING_LEVELS == 0 case.

Almost pure code movement - exceptions are the conversion of
"#if GUEST_PAGING_LEVELS == 2" to runtime conditionals and style
corrections (including to avoid open-coding mfn_to_maddr() and
PAGE_OFFSET()).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>

xen/arch/x86/mm/shadow/Makefile

@@ -1,5 +1,5 @@
 ifeq ($(CONFIG_SHADOW_PAGING),y)
-obj-y += common.o guest_2.o guest_3.o guest_4.o
+obj-y += common.o guest_2.o guest_3.o guest_4.o set.o
 obj-$(CONFIG_HVM) += hvm.o
 obj-$(CONFIG_PV) += pv.o
 else