git: add config file telling git-publish how to send patches

The "git-publish" tool is a useful git extension for sending patch
series for code review. It automatically creates versioned tags
each time code on a branch is sent, so that there is a record of
each version. It also remembers the cover letter so it does not
need re-entering each time the series is reposted.

With this config file present it is now sufficient[1] to run

  $ git publish

to send all patches in a branch to the list for review

[1] Assuming your $HOME/.gitconfig has an SMTP server listed
at least e.g.

   [sendemail]
        smtpserver = smtp.example.com
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitpublish

+[gitpublishprofile "default"]
+base = master
+to = libvir-list@redhat.com