s390x/pci: refactor s390_pci_find_dev_by_idx

s390_find_dev_by_idx() only indexes usable zpci devices. It implies
that the index value of each zpci device is dynamic and may change if
a new zpci device is plugged. So we have to use a constant index to
look up the device.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>