virsh: Don't output node frequency if unknown

Commit ed8155ea documented that
mhz field in virNodeInfo might be 0 if the frequency is unknown.  Modify
virsh to know about that.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

tools/virsh-host.c

@@ -637,7 +637,8 @@ cmdNodeinfo(vshControl *ctl, const vshCmd *cmd ATTRIBUTE_UNUSED)
     }
     vshPrint(ctl, "%-20s %s\n", _("CPU model:"), info.model);
     vshPrint(ctl, "%-20s %d\n", _("CPU(s):"), info.cpus);
-    vshPrint(ctl, "%-20s %d MHz\n", _("CPU frequency:"), info.mhz);
+    if (info.mhz)
+        vshPrint(ctl, "%-20s %d MHz\n", _("CPU frequency:"), info.mhz);
     vshPrint(ctl, "%-20s %d\n", _("CPU socket(s):"), info.sockets);
     vshPrint(ctl, "%-20s %d\n", _("Core(s) per socket:"), info.cores);
     vshPrint(ctl, "%-20s %d\n", _("Thread(s) per core:"), info.threads);

tools/virsh.pod

@@ -295,7 +295,8 @@ Print the XML representation of the hypervisor sysinfo, if available.
 Returns basic information about the node, like number and type of CPU,
 and size of the physical memory. The output corresponds to virNodeInfo
 structure. Specifically, the "CPU socket(s)" field means number of CPU
-sockets per NUMA cell.
+sockets per NUMA cell. The information libvirt displays is dependent
+upon what each architecture may provide.
 
 =item B<nodecpumap> [I<--pretty>]