Replace -no-virtio-balloon by -balloon

Replace -no-virtio-balloon by -balloon

We want to do (at least) two things to the virtio-balloon device:
suppress it, and control its PCI address.  Option -no-virtio-balloon
lets us do only the former.  To get the latter, replace
-no-virtio-balloon with

    -balloon none   disable balloon device
    -balloon virtio[,addr=str]
                    enable virtio balloon device (default)

Syntax suggested by Anthony Liguori.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>