Add ftp protocol support for cdrom disk

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>