IPI_TRACE is not really supported on xen

x86 stack_save_td_running() can work safely only if IPI_TRACE is a
non-maskable interrupt.  But at the moment FreeBSD/Xen does not provide
support for the NMI delivery mode.  So, mark the functionality as
unsupported similarly to other platforms without NMI.
Maybe there is a way to provide a Xen-specific working
stack_save_td_running(), but I couldn't figure it out.

MFC after:	3 weeks
Sponsored by:	Panzura

sys/x86/x86/stack_machdep.c

@@ -135,6 +135,13 @@ 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));