travis.yml: Enable builds on arm64, ppc64le and s390x

travis.yml: Enable builds on arm64, ppc64le and s390x

Travis recently added the possibility to test on these architectures,
too, so let's enable them in our travis.yml file to extend our test
coverage.

Unfortunately, the libssh in this Ubuntu version (bionic) is in a pretty
unusable Frankenstein state and libspice-server-dev is not available here,
so we can not use the global list of packages to install, but have to
provide individual package lists instead.

Also, some of the iotests crash when using "dist: bionic" on arm64
and ppc64le, thus these two builders have to use "dist: xenial" until
the problem is understood / fixed.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20191204154618.23560-8-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>