vmstate, memory: decouple vmstate from memory API

Currently creating a memory region automatically registers it for
live migration.  This differs from other state (which is enumerated
in a VMStateDescription structure) and ties the live migration code
into the memory core.

Decouple the two by introducing a separate API, vmstate_register_ram(),
for registering a RAM block for migration.  Currently the same
implementation is reused, but later it can be moved into a separate list,
and registrations can be moved to VMStateDescription blocks.

Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/hw/ppce500_mpc8544ds.c b/hw/ppce500_mpc8544ds.c
index 51b6abd..d69f78c 100644
--- a/hw/ppce500_mpc8544ds.c
+++ b/hw/ppce500_mpc8544ds.c
@@ -292,7 +292,8 @@
     ram_size &= ~(RAM_SIZES_ALIGN - 1);
 
     /* Register Memory */
-    memory_region_init_ram(ram, NULL, "mpc8544ds.ram", ram_size);
+    memory_region_init_ram(ram, "mpc8544ds.ram", ram_size);
+    vmstate_register_ram_global(ram);
     memory_region_add_subregion(address_space_mem, 0, ram);
 
     /* MPIC */