Add new domain device: "controller"

This augments virDomainDevice with a <controller> element
that is used to represent disk controllers (e.g., scsi
controllers). The XML format is given by

  <controller type="scsi" index="<num>">
     <address type="pci" domain="0xNUM" bus="0xNUM" slot="0xNUM"/>
  </controller>

where type denotes the disk interface (scsi, ide,...), index
is an integer that identifies the controller for association
with disks, and the <address> element specifies the controller
address on the PCI bus as described in previous commits
The address element can be omitted; in this case, an address
will be assigned automatically.

Most of the code in this patch is from Wolfgang Mauerer's
previous disk controller series

 * docs/schemas/domain.rng: Define syntax for <controller>
   XML element
 * src/conf/domain_conf.c, src/conf/domain_conf.h: Define
   virDomainControllerDef struct, and routines for parsing
   and formatting XML
* src/libvirt_private.syms: Add virDomainControllerInsert
   and virDomainControllerDefFree

docs/schemas/domain.rng

@@ -521,6 +521,25 @@
       </choice>
     </attribute>
   </define>
+  <define name="controller">
+    <element name="controller">
+      <optional>
+        <attribute name="type">
+          <choice>
+            <value>fdc</value>
+            <value>ide</value>
+            <value>scsi</value>
+          </choice>
+        </attribute>
+      </optional>
+      <attribute name="index">
+        <ref name="unsignedInt"/>
+      </attribute>
+      <optional>
+	<ref name="address"/>
+      </optional>
+    </element>
+  </define>
   <define name="filesystem">
     <element name="filesystem">
       <choice>
@@ -1225,6 +1244,7 @@
         <zeroOrMore>
           <choice>
             <ref name="disk"/>
+            <ref name="controller"/>
             <ref name="filesystem"/>
             <ref name="interface"/>
             <ref name="input"/>