It's possible to launch an instance with virtual interfaces
either by specifying the network the interface should be attached
to or the port.
There are cases (e.g. SR-IOV) where it's only possible to launch
an instance with a pre-created port.
Enabling the creation of a port with known vnic_types (Currently
'normal','direct' and 'macvtap') and launching an instance with this port
attached.
Nova support Neutron SR-IOV ports https://wiki.openstack.org/wiki/Nova-neutron-sriov
Change-Id: Id5cbd50d285ac0c7e8099c30151ace9d99f02020