Add new cover image type for Kobo driver

Kobo driver: Fix book covers not showing for sideloaded kepubs on the
book details screen, with newer Kobo firmware. Fixes #1260279 [Add new cover image type for Kobo driver](https://bugs.launchpad.net/calibre/+bug/1260279)

As reported http://www.mobileread.com/forums/showthread.php?t=228973,
the recent Kobo drivers have added new cover image type. This has been
on the Touch for a while, but only now seems to be being generated for
the newer ereaders.