x86/xen: add tty0 and hvc0 as preferred consoles for dom0

Today the tty0 and hvc0 consoles are added as a preferred consoles for
pv domUs only. As this requires a boot parameter for getting dom0
messages per default, add them for dom0, too.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>