KVM: Portability: Move kvm_vcpu definition back to kvm.h

This patches moves kvm_vcpu definition to kvm.h, and finally
kvm.h includes x86.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>