kvm tools: Don't always coalesce MMIOs

Coalescing MMIOs is only required when we really want avoid having to exit
to userspace for every MMIO write, usually it's not the case so no reason
to coalesce every MMIO zone (I was also pretty sure it didn't always coalesce
zones)..

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