kvm tools, mips: Adapt signature of kvm_cpu__emulate_io

Use struct kvm_cpu instead of struct kvm.
This change is req'd due to commit
8d770c4096cdf73e1b79e7395ef3a86aa2887077
(kvmtool: virtio: pass trapped vcpu to IO accessors).

Signed-off-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>