KVM: Rename kvm_arch_ops to kvm_x86_ops

This patch just renames the current (misnamed) _arch namings to _x86 to
ensure better readability when a real arch layer takes place.

Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>