Set kcons index to -1 (i.e., acquire all device indexes).

Set kcons index to -1 (i.e., acquire all device indexes).

This reverts part of the previous changeset, which prevented
console=tty0 from doing the obvious thing in a domU guest
(i.e., send output on the virtual console). This is because
the tty index ends up being 1.

I don't think setting the index to other than -1 really
buys us anything anyway. Any other driver that specifies
a name that conflicts with us will almost certainly set their
index to -1, so we cannot avoid conflicts by choosing a
more restrictive (singleton) value.

Signed-off-by: Keir Fraser <keir@xensource.com>