xen/netback: cleanup init and deinit code

Do some cleanup of the netback init and deinit code:

- add an omnipotent queue deinit function usable from
  xenvif_disconnect_data() and the error path of xenvif_connect_data()
- only install the irq handlers after initializing all relevant items
  (especially the kthreads related to the queue)
- there is no need to use get_task_struct() after creating a kthread
  and using put_task_struct() again after having stopped it.
- use kthread_run() instead of kthread_create() to spare the call of
  wake_up_process().

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <pdurrant@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>