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.args

@@ -4,6 +4,6 @@ unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -hda \
 /dev/HostVG/QEMUGuest1 -usb -spice port=5903,tls-port=5904,addr=127.0.0.1,\
 x509-dir=/etc/pki/libvirt-spice,tls-channel=main,plaintext-channel=inputs,\
 image-compression=auto_glz,jpeg-wan-compression=auto,zlib-glz-wan-compression=auto,\
-playback-compression=on,streaming-video=filter -vga \
+playback-compression=on,streaming-video=filter,disable-copy-paste -vga \
 qxl -global qxl.vram_size=18874368 -device qxl,id=video1,vram_size=33554432,bus=pci.0,addr=0x4 \
 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3

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'/>