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
Merge "Fix lack of extended port's attributes in Metaplugin"