kvm tools: add status notification hook to virtio-mmio

Patch e03b449cbddf (kvm tools: add status notification hook for
virtio) converted virtio-net to use a new notification mechanism,
but unfortunately left virtio-mmio behind.

The direct consequence is that both arm and arm64 guests are left
without any form of networking. Not good.

Bring virtio-mmio into this brave new notified world, and feel the
power of being able to ping again.

Acked-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Asias He <asias.hejun@gmail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>