Bug 857: Add workaround for a bug of old Open vSwitch.

Old version of OVS has a bug that it may clear OFPFW_DL_VLAN_PCP bit in
FLOW_REMOVED message incorrectly. This workaround tries to remove the SAL
Flow from the flow database after eliminating DL_VLAN_PR match field if
FLOW_REMOVED handler receives a SAL Flow that has DL_VLAN_PR match field.
This should not cause any problem because current VTN Manager never uses
DL_VLAN_PR match field.

Change-Id: I700c61a51459aa86609dde9d5b192a07bb0965d9
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>