autotest/autotest — pull request #227 by LaneWolf (2012-03-12 11:25:30+00:00)
Title: Introduce 'not_used' value for drive_cache option

Commit b6bb71130 fix using drive_cache option. However, this is not
supported by xen qemu and so breaks xen hvm installation.

This patch add special value - 'not_used'. When drive_cache has this value,
option is not added to installation command.

Signed-off-by: Miroslav Rezanina