revert r354482, checking for XENHVM was a wrong way of checking for Xen

svn path=/head/; revision=354489

sys/x86/x86/stack_machdep.c

@@ -135,13 +135,6 @@ int
 stack_save_td_running(struct stack *st, struct thread *td)
 {
 
-#ifdef XENHVM
-	/*
-	 * There is no NMI support on Xen, so this code can lead to
-	 * an inter-processor deadlock.
-	 */
-	return (EOPNOTSUPP);
-#endif /* XENHVM */
 #ifdef STACK
 	THREAD_LOCK_ASSERT(td, MA_OWNED);
 	MPASS(TD_IS_RUNNING(td));