xen-netfront: use netif_start_queue() on xennet_open()

xen-netfront never called netif_start_queue() and was was waking the
queue on xennet_open(), triggering the BUG_ON() on __netif_schedule().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>