Updating network interfaces returns 400 Bad Request

[alxiord]

PUT /network-interfaces before guest boot returns 400 when attempting to update the MAC.

curl --unix-socket /tmp/firecracker.socket -i \
      -X PUT "http://localhost/network-interfaces/1" \
      -H "accept: application/json" \
      -H "Content-Type: application/json" \
      -d "{ 
             \"iface_id\": \"1\", 
             \"host_dev_name\": \"vmtap33\", 
             \"guest_mac\": \"06:00:00:00:00:01\", 
             \"state\": \"Attached\" 
         }"
HTTP/1.1 201 Created
Content-Length: 0
Date: Fri, 15 Jun 2018 12:39:47 GMT

curl --unix-socket /tmp/firecracker.socket -i\
      -X PUT "http://localhost/network-interfaces/1"\
      -H "accept: application/json"\
      -H "Content-Type: application/json"\
      -d "{ 
            \"iface_id\": \"1\", 
            \"host_dev_name\": \"vmtap33\", 
            \"guest_mac\": \"06:00:00:00:00:02\", 
            \"state\": \"Attached\" 
        }"
HTTP/1.1 400 Bad Request
Content-Type: application/json
Transfer-Encoding: chunked
Date: Fri, 15 Jun 2018 12:39:52 GMT

{
  "fault_message": "Could not open TAP device."
}