[Network] Add support for MacVTap interfaces

I'm following the getting started guide, and trying to bind a macvtap to guest, which always give me this:
```
2020-05-28T14:03:41.904918179 [anonymous-instance:ERROR:src/api_server/src/parsed_request.rs:90] Received Error. Status code: 400 Bad Request. Message: Cannot open TAP device. Invalid name/permissions. CreateTap(Os { code: 22, kind: InvalidInput, message: Invalid argument })
```
I use following command to create macvtap and try to setup network for guest:
```
sudo ip link add link eno1 name macvtap0 type macvtap mode bridge
curl --unix-socket /tmp/firecracker.socket -i \
-X PUT 'http://localhost/network-interfaces/eth0' \
-H 'Accept: application/json' \
-H 'Content-Type: application/json' \
-d "{
\"iface_id\": \"eth0\",
\"host_dev_name\": \"macvtap0\"
}"
```
firecracker version: `0.21.1`