Always fetch back refs when reading vmi and use getattr instead of get..back_refs to avoid reading it again

Double commit from R1.06 commit https://github.com/Juniper/contrail-neutron-plugin/commit/819e5cf57c8ad4bffbc889dbb2357603384c86e3

Change-Id: Icfbbd6f36ca68a7da470e1840df8167337e3ead0
Closes-Bug: 1336716
Merge "Always fetch back refs when reading vmi and use getattr instead of get..back_refs to avoid reading it again"