docs: schemas: Decouple the virtio options from each other

Currently, all of the VirtioOptions are under a single <optional>
element, however, neither our parser/formatter or QEMU driver requires
the presence of all the options if only a single one from the set has
been specified, so fix it and silence the schema validator.
Signed-off-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

docs/schemas/domaincommon.rng

@@ -5498,6 +5498,8 @@
       <attribute name="iommu">
         <ref name="virOnOff"/>
       </attribute>
+    </optional>
+    <optional>
       <attribute name="ats">
         <ref name="virOnOff"/>
       </attribute>