xen-netback: move netback_probe() and netback_remove() to the end...

...of xenbus.c

This is a cosmetic function re-ordering to reduce churn in a subsequent
patch. Some style fix-up was done to make checkpatch.pl happier.

No functional change.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>