Netlink plugin: complete initialize structs used for testing.

Netlink plugin: complete initialize structs used for testing.

Valgrind complains about a conditional jump based on uninitialized
memory:

```
==66438== Conditional jump or move depends on uninitialised value(s)
==66438==    at 0x10CA06: vf_info_submit (in /__w/collectd/collectd/test_plugin_netlink)
==66438==    by 0x1110F2: test_vf_submit_test (in /__w/collectd/collectd/test_plugin_netlink)
==66438==    by 0x112EAC: main (in /__w/collectd/collectd/test_plugin_netlink)
```

This is likely caused by the `vf_stats_t` being only partially
initialized. Using a struct initializer is not only cleaner, it also
ensures the remainder of the struct is initialized to zero.