Fix DVR regression for ofagent

Fix DVR regression for ofagent

Background:
    ML2 plugin sometimes uses truncated port uuids.
    For example, in the case of ofagent and linuxbridge,
    if port id is 804ceaa1-0e3e-11e4-b537-08606e7f74e7,
    an agent would send "tap804ceaa1-0e" to the plugin.
    ML2 plugin's _device_to_port_id() would restore it to
    "804ceaa1-0e".  While it's still truncated, ML2 plugin's
    get_port() handles that by using "startswith".

The recently merged DVR change (https://review.openstack.org/#/c/102332/)
assumes that port_id is always a complete uuid (it's the case
for openvswitch) and fails to handle the above mentioned case.
This commit fixes the regression.

Change-Id: I9c0845be606969068ab5d13c0165e76760378500
Closes-Bug: #1343750