Add support for 9940x media

Reference:
http://mhvtl-linux-virtual-tape-library-community-forums.966029.n3.nabble.com/UNKNOWN-is-an-invalid-density-Warning-mamp-gt-MediaType-is-still-undefined-td4025317.html

Reported-by: nia
Signed-off-by: Mark Harvey <markh794@gmail.com>