KVM: Portability: Introduce kvm_vcpu_arch

Move all the architecture-specific fields in kvm_vcpu into a new struct
kvm_vcpu_arch.

Signed-off-by: Zhang Xiantao <xiantao.zhang@intel.com>
Acked-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>