datapath: Don't insert empty OVS_VPORT_ATTR_OPTIONS attribute

The port specific options are currently unused resulting in an
empty OVS_VPORT_ATTR_OPTIONS nested attribute being inserted
into every OVS_VPORT_CMD_GET message.

Don't insert OVS_VPORT_ATTR_OPTIONS if no options are present.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
[jesse: Options are used by tunnels but the concept still applies.]
Signed-off-by: Jesse Gross <jesse@nicira.com>