openvswitch: support setting OpenFlow datapath ID

openvswitch: support setting OpenFlow datapath ID

By default, Open vSwitch will generate the OpenFlow datapath ID of a
bridge based on the MAC address of one of its ports. Due to this, it's
possible that the datapath ID changes when new ports are added. When the
datapath ID changes, Open vSwitch disconnects from the controller, as
there is no way to notify the controller that the datapath ID has
changed.

Add an option to set the datapath ID so that the above situation can be
avoided. The option takes either exactly 16 hex characters, or when
prefixed with 0x, between 1 and 16 hex characters.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>