docs: Document configuration quirks for virtio devices

Some devices (namely virtio-scsi, virtio-gpu, virtio-keyboard,
virtio-tablet and virtio-mouse, plus virtio-crypto which is
not supported by libvirt) don't follow the same rules as all
other virtio devices, which is something that ought to be
documented.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>

docs/formatdomain.html.in

@@ -4132,6 +4132,24 @@
       </dd>
     </dl>
 
+    <p>
+      While the information outlined above applies to most virtio devices,
+      there are a few exceptions:
+    </p>
+
+    <ul>
+      <li>
+        for SCSI controllers, <code>virtio-scsi</code> must be used instead
+        of <code>virtio</code> for backwards compatibility reasons;
+      </li>
+      <li>
+        some devices, such as GPUs and input devices (keyboard, tablet and
+        mouse), are only defined in the virtio 1.0 spec and as such don't
+        have a transitional variant: the only accepted model is
+        <code>virtio</code>, which will result in a non-transitional device.
+      </li>
+    </ul>
+
     <p>
       For more details see the
       <a href="https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg00923.html">qemu patch posting</a> and the