form factor required for virtual interfaces

[hamishforbes]

Issue type

[ ] Feature request
[x] Bug report
[ ] Documentation

Environment

• Python version: 2.7.5
• NetBox version: 2.2-beta2

Description

Attempting to add an interface to a VM via the API fails if you don't specify the form_factor field.

The docs say the form_factor field is optional for POST /virtualization/interfaces/.
However unless that field is set to 0 or 200 (the Virtual type interfaces) an error is returned:

{
  "non_field_errors": [
    "Virtual machines cannot have physical interfaces."
  ]
}

This should probably default to 0?