migration: Move the VMStateDescription typedef to typedefs.h

We declare incomplete struct VMStateDescription in a couple of places
so we don't have to include migration/vmstate.h for the typedef.
That's fine with me.  However, the next commit will drop
migration/vmstate.h from a massive number of compiles.  Move the
typedef to qemu/typedefs.h now, so I don't have to insert struct in
front of VMStateDescription all over the place then.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190812052359.30071-15-armbru@redhat.com>