.travis.yml: add --disable-linux-user for some jobs

The modules and co-routine builds are only really relevant to softmmu
builds and regularly timeout on Travis. Let's disable linux-user
builds here for more headroom.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>