net: allow clients not associated with a vlan

net: allow clients not associated with a vlan

Allow net clients to be created which are not connected to any vlan.

This is needed by Gerd in order to allow adding -device nic, where
the absence of a vlan parameter will not imply vlan=0. Also needed
to allow adding a -netdevice option which doesn't connect the backend
to a vlan.

Patchworks-ID: 35513
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>