CI: Add a CentOS 6 container and build jobs

CI: Add a CentOS 6 container and build jobs

CentOS 6 is probably the most frequently broken build, so adding it to CI
would be a very good move.

One problem is that CentOS 6 comes with Python 2.6, and Qemu requires 2.7.
There appear to be no sensible ways to get Python 2.7 into a CentOS 6
environments, so modify the build script to skip the Qemu upstream build
instead.  Additionally, SeaBIOS requires GCC 4.6 or later, so skip it as well.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>