QEMU will soon drop the support for Ubuntu 18.04, so let's update
the Travis jobs that were still using this version to 20.04 instead.

While we're at it, also remove an obsolete comment about Ubuntu
Xenial being the default for our Travis jobs.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220221153423.1028465-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>