kvm tools: Add a void ptr to be passed to mmio callback

This makes MMIO callback similar to it's PIO counterpart by passing
a void* value provided in the registration to the callback function.

This allows to keep context within the MMIO callback function.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>