pseries: Fix reset of VIO network device

Currently, the PAPR VIO network device does not have a reset handler.  This
means that after a hard reset, H_REGISTER_LOGICAL_LAN will return an error
when the new guest boot attempts to initialize the device.

This patch corrects this, adding a suitable reset hook.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andreas Färber <afaerber@suse.de>
