s390x/pci: replace fid with idx in msg data of msix

Present code uses fid as the part of message data of msix for looking
up the specific zpci device. However it limits the usable range of fid,
and the code looking up the zpci device may fail due to truncation of
the fid.

In addition, fh is composed of enabled bit, FH_VIRT and the array index.
So we can use the array index as the identifier to store in msg data.

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>
