i40e: allow zero MAC address for VFs

Allow the user to specify a zero MAC address for VFs. This removes the
existing MAC address and allows the VF to use a random address. Libvirt
does this normally when removing a VF from a VM.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>