kvm tools: Add MMIO coalescing support

kvm tools: Add MMIO coalescing support

Coalescing MMIO allows us to avoid an exit every time we have a
MMIO write, instead - MMIO writes are coalesced in a ring which
can be flushed once an exit for a different reason is needed.
A MMIO exit is also trigged once the ring is full.

Coalesce all MMIO regions registered in the MMIO mapper.
Add a coalescing handler under kvm_cpu.

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