conf: Add channel devices to domain capabilities

As qemu becomes more modularized, it is important for libvirt to advertise
availability of the modularized functionality through capabilities. This
change adds channel devices to domain capabilities, allowing clients such
as virt-install to avoid using spicevmc channel devices when not supported
by the target qemu.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/domaincapsdata/qemu_5.0.0.ppc64.xml

@@ -143,6 +143,12 @@
       </enum>
     </tpm>
     <redirdev supported='no'/>
+    <channel supported='yes'>
+      <enum name='type'>
+        <value>pty</value>
+        <value>unix</value>
+      </enum>
+    </channel>
   </devices>
   <features>
     <gic supported='no'/>