xen/hvc: allow xenboot console to be used again

Commit d0380e6c

 (early_printk:
consolidate random copies of identical code) added in 3.10 introduced
a check for con->index == -1 in early_console_register().

Initialize index to -1 for the xenboot console so earlyprintk=xen
works again.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>