tools/kvm_stat: fix updates for dead guests

With pid filtering active, when a guest is removed e.g. via virsh shutdown,
successive updates produce garbage.
Therefore, we add code to detect this case and prevent further body updates.
Note that when displaying the help dialog via 'h' in this case, once we exit
we're stuck with the 'Collecting data...' message till we remove the filter.
Signed-off-by: NStefan Raspl <raspl@linux.ibm.com>
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>

tools/kvm/kvm_stat/kvm_stat

@@ -1170,6 +1170,9 @@ class Tui(object):
 
             return sorted_items
 
+        if not self._is_running_guest(self.stats.pid_filter):
+            # leave final data on screen
+            return
         row = 3
         self.screen.move(row, 0)
         self.screen.clrtobot()
@@ -1327,6 +1330,12 @@ class Tui(object):
                 msg = '"' + str(val) + '": Invalid value'
         self._refresh_header()
 
+    def _is_running_guest(self, pid):
+        """Check if pid is still a running process."""
+        if not pid:
+            return True
+        return os.path.isdir(os.path.join('/proc/', str(pid)))
+
     def _show_vm_selection_by_guest(self):
         """Draws guest selection mask.
 
@@ -1354,7 +1363,7 @@ class Tui(object):
             if not guest or guest == '0':
                 break
             if guest.isdigit():
-                if not os.path.isdir(os.path.join('/proc/', guest)):
+                if not self._is_running_guest(guest):
                     msg = '"' + guest + '": Not a running process'
                     continue
                 pid = int(guest)