QEMU/KVM already allows a HTTP URL for the cdrom ISO image so add this support
to libvirt as well.
The xml should be as following:

    <disk type='network' device='cdrom'>
      <source protocol='http' name='/url/path'>
        <host name='host.name' port='80'/>
      </source>
    </disk>
Signed-off-by: NAline Manera <alinefm@br.ibm.com>