new cmd line syntax:
    -chardev vc,id=name
    -chardev vc,id=name,width=pixels,height=pixels
    -chardev vc,id=name,cols=chars,rows=chars

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>