ts-xen-build: Pass --enable if --disable found in usage, and v.v.

ts-xen-build: Pass --enable if --disable found in usage, and v.v.

The existing code works in practice if the usage message always lists
the non-default, since the unlisted-in-usage options that would be
supported, but are elided, are in any case the default.

But configure might *compute* its defaults.  In which case it will
list only one of them in the usage message.  If the computed default
is not the same as the usual default (the one implied by listing the
opposite in the usage message) we would wrongly not pass the option.

So grep for both enable and disable.

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