media: Remove ISO header strings' padding

The strings in the ISO header ("volume-id", "system-id", "publisher-id"
and "application-id") have a predefined size and are padded with spaces,
but nothing in on_{pvd,svd}_read() removes these padding spaces from
those strings, which makes osinfo_media_get_volume_id() always return
[SOME-ISO-LABEL                  ] rather than the expected
[SOME-ISO-LABEL].

Resolves: rhbz#1408271

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>
Reported-by: Allan Day <aday@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>