netdev-dpdk: vhost-user port link state fix

netdev-dpdk: vhost-user port link state fix

OVS reports that link state of a vhost-user port (type=dpdkvhostuser) is
DOWN, even when traffic is running through the port between a Virtual
Machine and the vSwitch. Changing admin state with the
"ovs-ofctl mod-port <BR> <PORT> up/down" command over OpenFlow does
affect neither the reported link state nor the traffic.

The patch below does the flowing:
 - Triggers link state change by altering netdev's change_seq member.
 - Controls sending/receiving of packets through vhost-user port
   according to the port's current admin state.
 - Sets admin state of newly created vhost-user port to UP.

Signed-off-by: Zoltán Balogh <zoltan.balogh@ericsson.com>
Co-authored-by: Jan Scheurich <jan.scheurich@ericsson.com>
Signed-off-by: Jan Scheurich <jan.scheurich@ericsson.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>