conf: remove <backend upstream='xxx'/> attribute

conf: remove <backend upstream='xxx'/> attribute

This attribute was added to support setting the --interface option for
passt, but in a post-push/pre-9.0-release review, danpb pointed out
that it would be better to use the existing <source dev='xxx'/>
attribute to set --interface rather than creating a new attribute (in
the wrong place). So we remove backend/upstream, and change the passt
commandline creation to grab the name for --interface from source/dev.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>