s390x/pci: refactor list_pci

Because of the refactor of s390_pci_find_dev_by_idx(), list_pci()
should be updated. We introduce a new function to get the next
available zpci device. It simplifies the code of looking up zpci
devices.

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