Add 'type vnc' to vfb device sexpr for HVM guest

Add 'type vnc' to vfb device sexpr for HVM guest

This patch adds 'type vnc' to vfb device sexpr for HVM guests.  PV
guests already contain this entry in sexpr, e.g.

    (device
        (vfb
            (vncunused 1)
            ...
           (type vnc)
            ...
        )
    )

Some tools, such as libvirt, look for device/vfb/type in sexpr before
publishing VNC port.  More importantly, this patch provides
consistency in vfb device sexpr between HVM and PV guests.

Signed-off-by: Jim Fehlig <jfehlig@novell.com>