docs: add a note that spice channel is usable only with spice graphics

To prevent a confusion about missing chardev argument in qemu
command line add a note about that behavior into documentation.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1129198Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

docs/formatdomain.html.in

@@ -5380,6 +5380,11 @@ qemu-kvm -net nic,model=? /dev/null
       under a channel name specified in the <code>channel</code>
       attribute.  <span class="since">Since 1.2.2</span>
     </p>
+    <p>
+      Note: depending on the hypervisor, spiceports might (or might not)
+      be enabled on domains with or without <a href="#elementsGraphics">spice
+      graphics</a>.
+    </p>
 
 <pre>
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