netdev: get device type from vport prefix if it uses one

netdev: get device type from vport prefix if it uses one

If the device name uses a vport prefix, then use that vport type.

Since these names are reserved, we can assume this is the right type.

This is important when we are querying the datapath right after vswitch has
started and using the right type will be even more important when we add support
to creating tunnel ports with rtnetlink.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>