Fix lack of extended port's attributes in Metaplugin

Fix lack of extended port's attributes in Metaplugin

Previously, there are some extended port's attributes did not
appear in the result of GET port API when using Metaplugin.
This become a critial issue currently since lack of port binding
information disturbs normal port binding operation in a compute
node, for example.

Metaplugin did not delegate get_port/get_ports to target plugins.
This is cause of the problem because right plugin instance is not
passed to a hook which handles extended attributes.
Now, get_port/get_ports of target plugins are called so that
extended port's attributes are handled properly.

Change-Id: Ifed2505a97ceca56c4cedd22efa49adcdf3317b8
Closes-Bug: #1136252