travis-ci: Switch to building QEMU

First, there are a number of features in newer QEMU that will allow us
to test a wider range of platforms, so we want to use at least v2.8.0.
Second, making use of a PPA for QEMU fails from time to time.  So we
change to checking out and building a copy of QEMU when we know that we
are going to use test.py and need QEMU to be installed.  This adds
around 4 minutes per test.py job that we run.

Signed-off-by: Tom Rini <trini@konsulko.com>