kvm: specify nic_model explicitly for rtl8139

Instead of relying on the default nic_model, specify it explicitly. Different qemu branches use different defaults, and the default may change.

---
 client/tests/kvm/kvm_tests.cfg.sample |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index a83ef9b..aa6162f 100644
@@ -145,6 +145,7 @@ variants:
 # NICs
 variants:
     - @rtl8139:
+        nic_model = rtl8139
     - virtio:       rtl8139.install rtl8139.setup
         no install setup
         nic_model = virtio

1.6.4.1