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 <mprivozn@redhat.com>