pseries: Fix buggy spapr_vio_find_by_reg()

pseries: Fix buggy spapr_vio_find_by_reg()

The spapr_vio_find_by_reg() function in hw/spapr_vio.c is supposed to find
the device structure for a PAPR virtual IO device with the given reg value,
and return NULL if none exists.

It does the first ok, but if no device with that reg exists, it just
returns the last device traversed in the list.  This patch fixes it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>