conf: allow type='pci' addresses with no address attributes specified

Prior to this, <address type='pci'/> wasn't allowed when parsing
(domain+bus+slot+function needed to be a "valid" PCI address, meaning
that at least one of domain/bus/slot had to be non-0), the RNG
required bus to be specified, and if type was set to PCI when
formatting, domain+bus+slot+function would always be output.

This makes all the address attributes optional during parse and RNG
validation, and suppresses domain+bus+slot+function if domain+bus+slot
are all 0 (NB: if d+b+s are all 0, any value for function is
nonsensical as that will never happen in the real world, and after
the next patch we will always assign a real working address to any
empty PCI address before it is ever output to anywhere).

Note that explicitly setting all attributes to 0 is equivalent to
setting none of them, which is okay, since 0000:00:00 is reserved in
any PCI bus setup, and can't be used anyway.

docs/formatdomain.html.in

@@ -2930,7 +2930,13 @@
         (<span class="since">since 0.9.7, requires QEMU
         0.13</span>). <code>multifunction</code> defaults to 'off',
         but should be set to 'on' for function 0 of a slot that will
-        have multiple functions used.
+        have multiple functions used.<br/>
+        <span class="since">Since 1.3.5</span>, some hypervisor
+        drivers may accept an <code>&lt;address type='pci'/&gt;</code>
+        element with no other attributes as an explicit request to
+        assign a PCI address for the device rather than some other
+        type of address that may also be appropriate for that same
+        device (e.g. virtio-mmio).
       </dd>
       <dt><code>drive</code></dt>
       <dd>Drive addresses have the following additional

docs/schemas/basictypes.rng

@@ -83,9 +83,11 @@
         <ref name="pciDomain"/>
       </attribute>
     </optional>
-    <attribute name="bus">
-      <ref name="pciBus"/>
-    </attribute>
+    <optional>
+      <attribute name="bus">
+        <ref name="pciBus"/>
+      </attribute>
+    </optional>
     <optional>
       <attribute name="slot">
         <ref name="pciSlot"/>