Include migration/vmstate.h less

Include migration/vmstate.h less

In my "build everything" tree, changing migration/vmstate.h triggers a
recompile of some 2700 out of 6600 objects (not counting tests and
objects that don't depend on qemu/osdep.h).

hw/hw.h supposedly includes it for convenience.  Several other headers
include it just to get VMStateDescription.  The previous commit made
that unnecessary.

Include migration/vmstate.h only where it's still needed.  Touching it
now recompiles only some 1600 objects.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-Id: <20190812052359[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>