kvm tools: balloon: add dummy set_size_vq implementation

Commit 5e980d372620 ("kvm tools: virtio: add dummy set_size_vq
implementations") added dummy set_size_vq implementations for a number
of devices now that they can use virtio MMIO as their transport.

Unfortunately, it missed the balloon driver, so this patch adds the same
implementation there.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>