Description of problem
----------------------

Attempting to build a Fedora 23 image and updating its packages results
in `dnf update -y` failing with an AttributeError[1].  

    [...]
      File "/usr/lib/python3.4/site-packages/dnf/cli/cli.py", line 251, in gpgsigcheck
        if not sys.stdin.isatty() and not ay:
    AttributeError: 'NoneType' object has no attribute 'isatty'
    [...]

This seems like a `dnf` bug.  I'm filing it here since I noticed it in
the context of `virt-builder` (and a regular `dnf update -y` on the host
system works just fine.)


Version
-------

    $ uname -r
    4.2.5-300.fc23.x86_64

    $ rpm -q libguestfs-tools-c qemu-system-x86 libvirt-daemon-kvm dnf 
    libguestfs-tools-c-1.30.4-1.fc23.x86_64
    qemu-system-x86-2.4.1-1.fc23.x86_64
    libvirt-daemon-kvm-1.2.18.1-2.fc23.x86_64
    dnf-1.1.3-1.fc23.noarch


How reproducible: Consistently.


Steps to reproduce
------------------

Just invoke this command.

    $ virt-builder fedora-23 -o f23vm1.qcow2
        --format qcow2 --update --selinux-relabel
        --size 40G


Actual Results
--------------


    $ virt-builder fedora-23 -o f23vm1.qcow2 
        --format qcow2 --update --selinux-relabel 
        --size 40G
    [   1.5] Downloading: http://libguestfs.org/download/builder/fedora-23.xz
    ######################################################################## 100.0%
    [ 494.9] Planning how to build this image
    [ 494.9] Uncompressing
    [ 500.0] Resizing (using virt-resize) to expand the disk to 40.0G
    [ 528.1] Opening the new disk
    [ 545.2] Setting a random seed
    [ 545.2] Updating core packages
    
    [. . .]
    Delta RPMs reduced 93.7 MB of updates to 54.7 MB (41.1% saved)
    warning: /var/cache/dnf/updates-e042e478e0621ea6/packages/kernel-4.2.5-300.fc23.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID 34ec9cba: NOKEY
    The downloaded packages were saved in cache till the next successful transaction.
    You can remove cached packages by executing 'dnf clean packages'.
    Traceback (most recent call last):
      File "/bin/dnf", line 35, in <module>
        main.user_main(sys.argv[1:], exit_code=True)
      File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 198, in user_main
        errcode = main(args)
      File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 84, in main
        return _main(base, args)
      File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 144, in _main
        ret = resolving(cli, base)
      File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 173, in resolving
        base.do_transaction(display=displays)
      File "/usr/lib/python3.4/site-packages/dnf/cli/cli.py", line 220, in do_transaction
        self.gpgsigcheck(downloadpkgs)
      File "/usr/lib/python3.4/site-packages/dnf/cli/cli.py", line 251, in gpgsigcheck
        if not sys.stdin.isatty() and not ay:
    AttributeError: 'NoneType' object has no attribute 'isatty'
    virt-builder: no 'uri' entry for 'rhel' in /etc/xdg/virt-builder/repos.d/rhel.conf, skipping it
    virt-builder: error: dnf -y update: command exited with an error
    
    If reporting bugs, run virt-builder with debugging enabled and include the 
    complete output:
    
      virt-builder -v -x [...]


Expected results
----------------

`virt-builder --update [...]` should complete successfully.


Additional information
----------------------

- A regular `dnf update -y` on the host system itself works just fine.

- Simple image building (without '--update') works fine as well:

    $ virt-builder fedora-23 -o f23vm2.qcow2 --format qcow2


[1] https://docs.python.org/2/library/exceptions.html#exceptions.AttributeError