configs/qemu: Add multi-term.yaml

Extend the qemu/cca.yaml overlay with support for spawning multiple
terminals, which is very useful to launch commands in both host and
guest. This is enabled by adding the overlay directly to the run
command (not the build command):

  shrinkwrap run cca-3world.yaml -o qemu/multi-term.yaml

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>