balloon: add size information to memcpy fixup

I have recently introduced size calculation to memcpy fixup as a mean to make
sure that our moves are sane. However, that can't be a static function like I
have implemented.  This is because the rcx register holds number of iterations,
not number of bytes.  Therefore, we also need to include size information in
the fixup and make that a member function.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>