FIX: Add SRIOV cleanup

If Jenkins interrupt pybot during AVF tests (that is happening
more frequently as coverage increased) the interfaces remains
in link UP state. Next run VPP cannot grab interfaces as they
are in UP state in and kernel driver. This results in testcase
failing.

Patch is supposed to do put interfaces in topology down so VPP
can bind them back to vfio-pci.

Change-Id: I2aa9e5944d39474965eac3c5f1b472f58e9aa8b5
Signed-off-by: Peter Mikus <pmikus@cisco.com>