Fix libvirt test on systems with real iSCSI devices

The test_libvirt_kvm_iser_volume_with_multipath_getmpdev() test
expects a system with no real iSCSI devices. It actually triggers
an os.walk() of /dev/disk/by-path/ip-*, which will return any
iSCSI devices on the system. Since the test expects none, this
will cause the test to fail.

This patch just stubs that out to return an empty list always.

Change-Id: Ibb3fd18207d013c35487a3eebb336b5f288b703a
Closes-bug: #1234834
Merge "Fix libvirt test on systems with real iSCSI devices"