Add ftp protocol support for cdrom disk

The ftp protocol is already recognized by qemu/KVM so add this support to
libvirt as well.
The xml should be as following:

     <disk type='network' device='cdrom'>
       <source protocol='ftp' name='/url/path'>
         <host name='host.name' port='21'/>
       </source>
     </disk>

Signed-off-by: Aline Manera <alinefm@br.ibm.com>

docs/formatdomain.html.in

@@ -1509,6 +1509,14 @@
       <target dev='hdc' bus='ide' tray='open'/>
       <readonly/>
     </disk>
+    <disk type='network' device='cdrom'>
+      <driver name='qemu' type='raw'/>
+      <source protocol="ftp" name="url_path">
+        <host name="hostname" port="21"/>
+      </source>
+      <target dev='hdc' bus='ide' tray='open'/>
+      <readonly/>
+    </disk>
     <disk type='block' device='lun'>
       <driver name='qemu' type='raw'/>
       <source dev='/dev/sda'/>

docs/schemas/domaincommon.rng

@@ -1212,6 +1212,7 @@
         <value>gluster</value>
         <value>iscsi</value>
         <value>http</value>
+        <value>ftp</value>
       </choice>
     </attribute>
     <optional>