s390/pci: Improve handling of unset UID

s390/pci: Improve handling of unset UID

When UID checking is enabled a UID value of 0 is invalid and can not be
set by the user. On z/VM it is however used to indicate an unset UID.
Until now, this lead to the behavior that one PCI function could be
attached with UID 0 after which z/VM would prohibit further attachment.

Now if the user then turns off UID checking in z/VM the user could
seemingly attach additional PCI functions that would however not show up
in Linux as that would not be informed of the change in UID checking
mode. This is unexpected and confusing and lead to bug reports against
Linux.

Instead now, if we encounter an unset UID value of 0 treat it as
indicating that UID checking was turned off, switch to automatic domain
allocation, and warn the user of the possible misconfiguration.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>