conf: add 'sharePolicy' attribute to graphics element for vnc

-vnc :5900,share=allow-exclusive
allows clients to ask for exclusive access which is
implemented by dropping other connections Connecting
multiple clients in parallel requires all clients asking
for a shared session (vncviewer: -shared switch)

 -vnc :5900,share=force-shared
disables exclusive client access.  Useful for shared
desktop sessions, where you don't want someone forgetting
specify -shared disconnect everybody else.

 -vnc :5900,share=ignore
completely ignores the shared flag and allows everybody
connect unconditionally

docs/formatdomain.html.in

@@ -3621,7 +3621,7 @@ qemu-kvm -net nic,model=? /dev/null
   ...
   <devices>
     <graphics type='sdl' display=':0.0'/>
-    <graphics type='vnc' port='5904'>
+    <graphics type='vnc' port='5904' sharePolicy='allow-exclusive'>
       <listen type='address' address='1.2.3.4'/>
     </graphics>
     <graphics type='rdp' autoport='yes' multiUser='yes' />
@@ -3664,7 +3664,16 @@ qemu-kvm -net nic,model=? /dev/null
             allows control of connected client during password changes.
             VNC accepts <code>keep</code> value only.
             <span class="since">since 0.9.3</span>
-            NB, this may not be supported by all hypervisors.<br/>  <br/>
+            NB, this may not be supported by all hypervisors.<br/>
+            The optional <code>sharePolicy</code> attribute specifies vnc server
+            display sharing policy. "allow-exclusive" allows clients to ask
+            for exclusive access by dropping other connections. Connecting
+            multiple clients in parallel requires all clients asking for a
+            shared session (vncviewer: -Shared switch). This is the default
+            value. "force-shared" disables exclusive client access, every
+            connection has to specify -Shared switch for vncviewer. "ignore"
+            welcomes every connection unconditionally
+            <span class="since">since 1.0.6</span>. <br/> <br/>
             Rather than using listen/port, QEMU supports a
             <code>socket</code> attribute for listening on a unix
             domain socket path.<span class="since">Since 0.8.8</span>

docs/schemas/domaincommon.rng

@@ -2135,6 +2135,15 @@
                   <ref name="addrIPorName"/>
                 </attribute>
               </optional>
+              <optional>
+                <attribute name='sharePolicy'>
+                  <choice>
+                    <value>allow-exclusive</value>
+                    <value>force-shared</value>
+                    <value>ignore</value>
+                  </choice>
+                </attribute>
+              </optional>
             </group>
             <group>
               <optional>