osstest: explicitly set either enable or disable qemu-traditional

osstest: explicitly set either enable or disable qemu-traditional

Instead of setting "--enable-qemu-traditional" or not, switch to
setting "--enable-qemu-traditional" or "--disable-qemu-traditional".

This avoids a latent bug in the disable case, as the availability
of the option is tested via grep, which will otherwise grep for
nothing.

Reported-by: Ian Jackson <iwj@xenproject.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>