KVM: x86: Check LMA bit before set_efer

kvm_x86_ops->set_efer() would execute vcpu->arch.efer = efer, so the
checking of LMA bit didn't work.

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>