add qemu_add_data_dir()

Add helper function to add a directory to the qemu search path, so we
don't duplicate the checks.  Add a check for duplicate entries, so we
stop trying to open files twice.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170914114236.25343-2-kraxel@redhat.com
