memory: Switch memory from using AddressSpace to FlatView

FlatView's will be shared between AddressSpace's and subpage_t
and MemoryRegionSection cannot store AS anymore, hence this change.

In particular, for:

 typedef struct subpage_t {
     MemoryRegion iomem;
-    AddressSpace *as;
+    FlatView *fv;
     hwaddr base;
     uint16_t sub_section[];
 } subpage_t;

  struct MemoryRegionSection {
     MemoryRegion *mr;
-    AddressSpace *address_space;
+    FlatView *fv;
     hwaddr offset_within_region;
     Int128 size;
     hwaddr offset_within_address_space;
     bool readonly;
 };

This should cause no behavioural change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-7-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>