Description of problem:
When doing yum operations it's not possible to exclude the @System virtual repository.

Version-Release number of selected component (if applicable):
dnf-1.0.1-2.fc22.noarch

How reproducible:
Reproducible every time.

Steps to Reproduce:
1. Do a dnf info on a package that is installed e.g.
dnf info dnf
2. Note that multiple packages are returned.
3. Retry the search with all repositories disabled:
dnf --disablerepo='*' info dnf

Actual results:
Installed Packages
Name        : dnf
Arch        : noarch
Epoch       : 0
Version     : 1.0.1
Release     : 2.fc22
Size        : 714 k
Repo        : @System
From repo   : updates
Summary     : Package manager forked from Yum, using libsolv as a dependency resolver
URL         : https://github.com/rpm-software-management/dnf
License     : GPLv2+ and GPLv2 and GPL
Description : Package manager forked from Yum, using libsolv as a dependency resolver.

Expected results:
No information to be returned.

Additional info:
Trying to do
dnf --disablerepo='@System' info dnf
results in
Error: Unknown repo: '@System'