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/vmware_vga.c b/hw/vmware_vga.c
index af70bde..b1885c3 100644
--- a/hw/vmware_vga.c
+++ b/hw/vmware_vga.c
@@ -1091,7 +1091,8 @@
 
 
     s->fifo_size = SVGA_FIFO_SIZE;
-    memory_region_init_ram(&s->fifo_ram, NULL, "vmsvga.fifo", s->fifo_size);
+    memory_region_init_ram(&s->fifo_ram, "vmsvga.fifo", s->fifo_size);
+    vmstate_register_ram_global(&s->fifo_ram);
     s->fifo_ptr = memory_region_get_ram_ptr(&s->fifo_ram);
 
     vga_common_init(&s->vga, vga_ram_size);