KVM: nSVM: check for EFER.SVME=1 before entering guest

EFER is set for L2 using svm_set_efer, which hardcodes EFER_SVME to 1 and hides
an incorrect value for EFER.SVME in the L1 VMCB.  Perform the check manually
to detect invalid guest state.
Reported-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -3558,6 +3558,9 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
 
 static bool nested_vmcb_checks(struct vmcb *vmcb)
 {
+	if ((vmcb->save.efer & EFER_SVME) == 0)
+		return false;
+
 	if ((vmcb->control.intercept & (1ULL << INTERCEPT_VMRUN)) == 0)
 		return false;