virpcitest: Fix variable arguments using in pci_driver_new

virpcitest: Fix variable arguments using in pci_driver_new

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: Michal Privoznik <mprivozn@redhat.com>