Use monospace font for log viewer.

logdlg.ui

@@ -16,6 +16,11 @@
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QTextBrowser" name="log_text">
+     <property name="font">
+      <font>
+       <family>Monospace</family>
+      </font>
+     </property>
      <property name="toolTip">
       <string>Copy Dom0 clipboard to Qubes clipboard</string>
      </property>