Use xcall to suspend/resume clocks on _all_ CPUs, not just current.

Index: sys/arch/xen/xen/xen_machdep.c
===================================================================
RCS file: /cvsroot/src/sys/arch/xen/xen/xen_machdep.c,v
retrieving revision 1.16
diff -p -u -r1.16 xen_machdep.c
--- sys/arch/xen/xen/xen_machdep.c	24 Jun 2018 20:28:58 -0000	1.16
+++ sys/arch/xen/xen/xen_machdep.c	30 Jun 2018 14:35:48 -0000
@@ -279,6 +279,9 @@ sysctl_xen_suspend(SYSCTLFN_ARGS)
 
 }
 
+static xcfunc_t xen_suspendclocks_xc;
+static xcfunc_t xen_resumeclocks_xc;
+
 /*
  * Last operations before suspending domain
  */
@@ -289,7 +292,7 @@ xen_prepare_suspend(void)
 	kpreempt_disable();
 
 	pmap_xen_suspend();
-	xen_suspendclocks(curcpu());
+	xc_wait(xc_broadcast(0, &xen_suspendclocks_xc, NULL, NULL));
 
 	/*
 	 * save/restore code does not translate these MFNs to their
@@ -312,6 +315,15 @@ xen_prepare_suspend(void)
 
 }
 
+static void
+xen_suspendclocks_xc(void *a, void *b)
+{
+
+	kpreempt_disable();
+	xen_suspendclocks(curcpu());
+	kpreempt_enable();
+}
+
 /*
  * First operations before restoring domain context
  */
@@ -342,13 +354,22 @@ xen_prepare_resume(void)
 
 	xen_suspend_allow = false;
 
-	xen_resumeclocks(curcpu());
+	xc_wait(xc_broadcast(0, xen_resumeclocks_xc, NULL, NULL));
 
 	kpreempt_enable();
 
 }
 
 static void
+xen_resumeclocks_xc(void *a, void *b)
+{
+
+	kpreempt_disable();
+	xen_resumeclocks(curcpu());
+	kpreempt_enable();
+}
+
+static void
 xen_suspend_domain(void)
 {
 	paddr_t mfn;