graphics: add support for action_if_connected in qemu

This option accepts 3 values:
-keep, to keep current client connected (Spice+VNC)
-disconnect, to disconnect client (Spice)
-fail, to fail setting password if there is a client connected (Spice)

docs/schemas/domain.rng

@@ -1294,6 +1294,13 @@
               <data type="dateTime"/>
             </attribute>
           </optional>
+          <optional>
+            <attribute name="connected">
+              <choice>
+                <value>keep</value>
+              </choice>
+            </attribute>
+          </optional>
         </group>
         <group>
           <attribute name="type">
@@ -1337,6 +1344,15 @@
               <data type="dateTime"/>
             </attribute>
           </optional>
+          <optional>
+            <attribute name="connected">
+              <choice>
+                <value>fail</value>
+                <value>disconnect</value>
+                <value>keep</value>
+              </choice>
+            </attribute>
+          </optional>
           <interleave>
             <zeroOrMore>
               <element name="channel">