KVM: Portability: Move kvm_vcpu_stat to x86.h

This patches moves kvm_vcpu_stat to x86.h, so every
arch can define its own kvm_vcpu_stat structure.

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>