xl: Return an error on failed cd-insert

xl: Return an error on failed cd-insert

This makes xl more useful in scripts.

The strange thing about this is that the internal cd_insert function
*already* returned something appropriate, and cd-eject was using it,
but cd-insert wasn't.

Also:

* Rework cd_insert to return EXIT_FAILURE and EXIT_SUCCESS rather than
magic constants

* Use 'r' for non-libxl return code, as specified in CODING_STYLE

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>