Configure the tap fd before passing it to crosvm

Crosvm doesn't enforce or check any configuration on the tap file
descriptors it gets for networking, so the required configuration
needs to be set before calling the VMM.
This was not a problem with qemu because it actually enforces the
configuration it requires.

Bug: 127708997
Test: run locally
Change-Id: I4979a1d9a60c4dfa6f62f2fec83968c447f8862a