schema: make pci slot and function optional

The pcie-switch-downstream-port and pcie-root-port controllers have
only a single slot, numbered 0, and the greate majority of all guest
PCI devices are plugged into function 0 of whatever slot they're
using. The parser makes these optional, setting them to 0 when not
specified, and it's logical for the schema to also make them optional.

docs/schemas/basictypes.rng

@@ -75,12 +75,16 @@
     <attribute name="bus">
       <ref name="pciBus"/>
     </attribute>
+    <optional>
       <attribute name="slot">
         <ref name="pciSlot"/>
       </attribute>
+    </optional>
+    <optional>
       <attribute name="function">
         <ref name="pciFunc"/>
       </attribute>
+    </optional>
     <optional>
       <attribute name="multifunction">
         <ref name="virOnOff"/>