s390/pci: track whether util_str is valid in the zpci_dev

We'll need to keep track of whether or not the byte string in util_str is
valid and thus needs to be passed to a vfio-pci passthrough device.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Niklas Schnelle <schnelle@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>