In the pci_driver_new function it is possible to set a list of
<vendor:device> IDs that the driver knows. These IDs are passed as
variable arguments and are processed  the usual way using va_start() and
va_arg(). However, after all arguments has been processed, we should
call va_end() what we aren't currently doing.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

This commit introduces yet another test under virpcitest:
virPCIDeviceDetach. However, in order to be able to do this, the
virpcimock needs to be extended to model the kernel behavior on PCI
device binding and unbinding (create 'driver' symlinks under the device
tree, check for device ID in driver's ID table, etc.)
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

Among with this test introduce virpcimock as we need to mock some
syscalls, e.g. redirect open() of a file under /sys/bus/pci to a
stub sysfs tree.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>