KVM: x86: Call vcpu_load and vcpu_put in cpuid_update

cpuid_update may operate VMCS, so vcpu_load() and vcpu_put()
should be called to ensure correctness.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>