Fixed crash with selftest when multiple PCI devices are available.

The selftest will execute for each suitable PCI device that is
found. As part of instantiating an instance of the intel NIC, the ffi
cdefs are executed twice causing a crash due to the attemp to redefine
already existing cdefs.

See
https://github.com/SnabbCo/snabbswitch/issues/34#issuecomment-12409461
for a sample stack trace that is generated due to the issue.