gitpublish: add a subject prefix

Now that we removed the subject prefix tag from the mailman config
we should set 'libvirt' as the subject when sending patches.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitpublish

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