travis: trim out most clang builds

We test with both gcc and clang in order to detect cases
where clang issues warnings that gcc misses. To achieve
this though we don't need to build QEMU in multiple
different configurations. Just a single clang-on-linux
build will be sufficient, if we have an "all enabled"
config.

This cuts the number of build jobs from 21 to 16,
reducing the load imposed on shared Travis CI infra.
This will make it practical to enable jobs for other
interesting & useful configurations without DOS'ing
Travis to much.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
