This will allow meson to honour -Dauto_features=disabled later.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201210190417.31673-4-alex.bennee@linaro.org>