Introduce new XMLs to specify disk source using libvirt storage

With this patch, one can specify the disk source using libvirt
storage like:

  <disk type='volume' device='disk'>
    <driver name='qemu' type='raw' cache='none'/>
    <source pool='default' volume='fc18.img'/>
    <target dev='vdb' bus='virtio'/>
  </disk>

"seclabels" and "startupPolicy" are not supported for this new
disk type ("volume"). They will be supported in later patches.

docs/formatdomain.html.in:
  * Add documents for new XMLs
docs/schemas/domaincommon.rng:
  * Add rng for new XMLs;
src/conf/domain_conf.h:
  * New struct for 'volume' type disk source (virDomainDiskSourcePoolDef)
  * Add VIR_DOMAIN_DISK_TYPE_VOLUME for enum virDomainDiskType
src/conf/domain_conf.c:
  * New helper virDomainDiskSourcePoolDefParse to parse the 'volume'
    type disk source.
  * New helper virDomainDiskSourcePoolDefFree to free the source def
    if 'volume' type disk.
tests/qemuxml2argvdata/qemuxml2argv-disk-source-pool.xml:
tests/qemuxml2xmltest.c:
  * New test

docs/formatdomain.html.in

@@ -1372,6 +1372,11 @@
       <blockio logical_block_size='512' physical_block_size='4096'/>
       <target dev='hda' bus='ide'/>
     </disk>
+    <disk type='volume' device='disk'>
+      <driver name='qemu' type='raw'/>
+      <source pool='blk-pool0' volume='blk-pool0-vol0'/>
+      <target dev='hda' bus='ide'/>
+    </disk>
   </devices>
   ...</pre>
 
@@ -1452,10 +1457,16 @@
         <code>iqn.1992-01.com.example/1</code>); the default LUN is zero.
         When the disk <code>type</code> is "network", the <code>source</code>
         may have zero or more <code>host</code> sub-elements used to
-        specify the hosts to connect.
+        specify the hosts to connect.  If the disk <code>type</code> is
+        "volume", the underlying disk source is represented by attributes
+        <code>pool</code> and <code>volume</code>. Attribute <code>pool</code>
+        specifies the name of storage pool (managed by libvirt) where the disk
+        source resides, and attribute <code>volume</code> specifies the name of
+        storage volume (managed by libvirt) used as the disk source.
         <span class="since">Since 0.0.3; <code>type='dir'</code> since
         0.7.5; <code>type='network'</code> since
-        0.8.7; <code>protocol='iscsi'</code> since 1.0.4</span><br/>
+        0.8.7; <code>protocol='iscsi'</code> since 1.0.4;
+        <code>type='volume'</code> since 1.0.5;</span><br/>
         For a "file" disk type which represents a cdrom or floppy
         (the <code>device</code> attribute), it is possible to define
         policy what to do with the disk if the source file is not accessible.

docs/schemas/domaincommon.rng

@@ -1107,6 +1107,24 @@
             <ref name="diskspec"/>
           </interleave>
         </group>
+        <group>
+          <attribute name="type">
+            <value>volume</value>
+          </attribute>
+          <interleave>
+            <optional>
+              <element name="source">
+                <attribute name="pool">
+                  <ref name="genericName"/>
+                </attribute>
+                <attribute name="volume">
+                  <ref name="volName"/>
+                </attribute>
+              </element>
+            </optional>
+            <ref name="diskspec"/>
+          </interleave>
+        </group>
         <ref name="diskspec"/>
       </choice>
     </element>