KVM: Portability: Move kvm_vm_stat to x86.h

This patch moves kvm_vm_stat to x86.h, and every arch
can define its own kvm_vm_stat in $arch.h

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>