spice: add <clipboard copypaste='yes|no'> option

From a security pov copy and paste between the guest and the client is not
always desirable. So we need to be able to enable/disable this. The best place
to do this from an administration pov is on the hypervisor, so the qemu cmdline
is getting a spice disable-copy-paste option, see bug 693645. Example qemu
invocation:
qemu -spice port=5932,disable-ticketing,disable-copy-paste

https://bugzilla.redhat.com/show_bug.cgi?id=693661

tests/qemuxml2argvdata/qemuxml2argv-graphics-spice.xml

@@ -29,6 +29,7 @@
       <zlib compression='auto'/>
       <playback compression='on'/>
       <streaming mode='filter'/>
+      <clipboard copypaste='no'/>
     </graphics>
     <video>
       <model type='qxl' vram='18432' heads='1'/>