kvm tools: Fix MMIO ordering problem cause due to coalescing

If we took a MMIO exit, the data in the coalesced ring should be processes
before the data in the exit itself is processed.

Doing it wrong (like we did so far) will cause ordering issues.

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