kvm tools: Create arch-specific kvm_cpu__emulate_{mm}io()

Different architectures will deal with MMIO exits differently.  For example,
KVM_EXIT_IO is x86-specific, and I/O cycles are often synthesised by steering
into windows in PCI bridges on other architectures.

This patch calls arch-specific kvm_cpu__emulate_io() and kvm_cpu__emulate_mmio()
from the main runloop's IO and MMIO exit handlers.  For x86, these directly
call kvm__emulate_io() and kvm__emulate_mmio() but other architectures will
perform some address munging before passing on the call.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>