libxl: add support for memballoon device

All Xen PV and HVM with PV driver support a memory balloon device,
which cannot be disabled through the toolstack. Model the device
in the libxl driver, similar to the recently removed xend-based
driver.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/sexpr2xmldata/sexpr2xml-fv-serial-pty.xml

@@ -52,5 +52,6 @@
     <video>
       <model type='cirrus' vram='8192' heads='1' primary='yes'/>
     </video>
+    <memballoon model='xen'/>
   </devices>
 </domain>