libxl: Don't use tapdisk for cd-roms

libxl: Don't use tapdisk for cd-roms

blktap does not support the insert / eject commands, and so is not
suitable for cd-roms.

This fixes the bug where libxl uses tapdisk as a cdrom back-end, causing
subsequent eject / insert commands to fail.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
CC: Fabio Fantoni <fabio.fantoni@heliman.it>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>