bhyve: fix crash in bhyveBuildNetArgStr

bhyve: fix crash in bhyveBuildNetArgStr

bhyveBuildNetArgStr() calls virNetDevTapCreateInBridgePort() and
passes tapfd = NULL, but tapfdSize = 1. That is wrong, because
if virNetDevTapCreateInBridgePort() crashes after successfully
creating a TAP device, it'll jump to 'error' label, that
loops over tapfd and calls VIR_FORCE_CLOSE:

   for (i = 0; i < tapfdSize && tapfd[i] >= 0; i++)

In that case we get a segfault.

As the bhyve code doesn't use tapfd, pass NULL and set tapfdSize to 0.