x86/Xen: further refine add_preferred_console() invocations

As the sequence of invocations matters, add "tty" only after "hvc" when
a VGA console is available (which is often the case for Dom0, but hardly
ever for DomU).

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