storage: ZFS support

Implement ZFS storage backend driver. Currently supported
only on FreeBSD because of ZFS limitations on Linux.

Features supported:

 - pool-start, pool-stop
 - pool-info
 - vol-list
 - vol-create / vol-delete

Pool definition looks like that:

 <pool type='zfs'>
  <name>myzfspool</name>
  <source>
    <name>actualpoolname</name>
  </source>
 </pool>

The 'actualpoolname' value is a name of the pool on the system,
such as shown by 'zpool list' command. Target makes no sense
here because volumes path is always /dev/zvol/$poolname/$volname.

User has to create a pool on his own, this driver doesn't
support pool creation currently.

A volume could be used with Qemu by adding an entry like this:

    <disk type='volume' device='disk'>
      <driver name='qemu' type='raw'/>
      <source pool='myzfspool' volume='vol5'/>
      <target dev='hdc' bus='ide'/>
    </disk>

docs/schemas/storagepool.rng

@@ -22,6 +22,7 @@
         <ref name='poolrbd'/>
         <ref name='poolsheepdog'/>
         <ref name='poolgluster'/>
+        <ref name='poolzfs'/>
       </choice>
     </element>
   </define>
@@ -157,6 +158,17 @@
     </interleave>
   </define>
 
+  <define name='poolzfs'>
+    <attribute name='type'>
+      <value>zfs</value>
+    </attribute>
+    <interleave>
+      <ref name='commonmetadata'/>
+      <ref name='sizing'/>
+      <ref name='sourcezfs'/>
+    </interleave>
+  </define>
+
   <define name='sourceinfovendor'>
     <interleave>
       <optional>
@@ -370,6 +382,14 @@
     </element>
   </define>
 
+  <define name='sourcezfs'>
+    <element name='source'>
+      <interleave>
+        <ref name='sourceinfoname'/>
+      </interleave>
+    </element>
+  </define>
+
   <define name='sourcefmtfs'>
     <optional>
       <element name='format'>

docs/storage.html.in

@@ -117,6 +117,9 @@
       <li>
         <a href="#StorageBackendGluster">Gluster backend</a>
       </li>
+      <li>
+        <a href="#StorageBackendZFS">ZFS backend</a>
+      </li>
     </ul>
 
     <h2><a name="StorageBackendDir">Directory pool</a></h2>
@@ -743,5 +746,36 @@
       pool type.
     </p>
 
+    <h2><a name="StorageBackendZFS">ZFS pools</a></h2>
+    <p>
+      This provides a pool based on the ZFS filesystem. It is currently
+      supported on FreeBSD only.
+
+      A pool has to be created before libvirt could start using it. That
+      could be done using <code>zpool create</code> command. Please refer to
+      the ZFS documentation for details on a pool creation.
+
+      <span class="since">Since 1.2.8</span>
+    </p>
+
+    <h3>Example pool input</h3>
+    <pre>
+      &lt;pool type="zfs"&gt;
+        &lt;name&gt;myzfspool&lt;/name&gt;
+        &lt;source&gt;
+          &lt;name&gt;zpoolname&lt;/name&gt;
+        &lt;/source&gt;
+       &lt;/pool&gt;</pre>
+
+    <h3>Valid pool format types</h3>
+    <p>
+      The ZFS volume pool does not use the pool format type element.
+    </p>
+
+    <h3>Valid pool format types</h3>
+    <p>
+      The ZFS volume pool does not use the volume format type element.
+    </p>
+
   </body>
 </html>