virpci: Don't error on unbinded devices
virpci: Don't error on unbinded devices

https://bugzilla.redhat.com/show_bug.cgi?id=1018897

If a PCI deivce is not binded to any driver (e.g. there's yet no PCI
driver in the linux kernel) but still users want to passthru the device
we fail the whole operation as we fail to resolve the 'driver' link
under the PCI device sysfs tree. Obviously, this is not a fatal error
and it shouldn't be error at all.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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