libvirt: add todo about bdms in _build_device_metadata
libvirt: add todo about bdms in _build_device_metadata

This came up while reviewing this code, but the TODO
wasn't added. Since the BDM lookup impacts performance
and there is a solution to avoid that, this adds the
TODO so we don't forget about it.

Related to blueprint virt-device-role-tagging

Change-Id: I87c2d5434bf2c40757bc1ea777d593e49e6c33b2

rajalokan/nova,jianghuaw/nova,phenoxim/nova,mikalstill/nova,klmitch/nova,rajalokan/nova,Juniper/nova,mikalstill/nova,rajalokan/nova,vmturbo/nova,jianghuaw/nova,cloudbase/nova,vmturbo/nova,cloudbase/nova,Juniper/nova,klmitch/nova,mikalstill/nova,rahulunair/nova,Juniper/nova,rahulunair/nova,vmturbo/nova,mahak/nova,jianghuaw/nova,klmitch/nova,gooddata/openstack-nova,Juniper/nova,vmturbo/nova,klmitch/nova,rajalokan/nova,gooddata/openstack-nova,sebrandon1/nova,jianghuaw/nova,hanlind/nova,sebrandon1/nova,phenoxim/nova,openstack/nova,gooddata/openstack-nova,alaski/nova,gooddata/openstack-nova,mahak/nova,alaski/nova,openstack/nova,hanlind/nova,hanlind/nova,rahulunair/nova,sebrandon1/nova,cloudbase/nova,openstack/nova,mahak/nova