tools/kvm_stat: print error on invalid regex

Entering an invalid regular expression did not produce any indication of an
error so far.
To reproduce, press 'f' and enter 'foo(' (with an unescaped bracket).
Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/kvm/kvm_stat/kvm_stat

@@ -1176,6 +1176,7 @@ class Tui(object):
         Asks for a valid regex and sets the fields filter accordingly.
 
         """
+        msg = ''
         while True:
             self.screen.erase()
             self.screen.addstr(0, 0,
@@ -1184,6 +1185,7 @@ class Tui(object):
             self.screen.addstr(2, 0,
                                "Current regex: {0}"
                                .format(self.stats.fields_filter))
+            self.screen.addstr(5, 0, msg)
             self.screen.addstr(3, 0, "New regex: ")
             curses.echo()
             regex = self.screen.getstr().decode(ENCODING)
@@ -1198,6 +1200,7 @@ class Tui(object):
                 self.refresh_header()
                 return
             except re.error:
+                msg = '"' + regex + '": Not a valid regular expression'
                 continue
 
     def show_vm_selection_by_pid(self):