qemu: Add run_iozone.py example

Add an example of test that runs the iozone benchmark on
a qemu vm over a range of qemu commits. Add a multiplex
file that supports the test execution, through the command
line:

avocado run qemu/run_iozone.py --multiplex qemu/run_iozone.py.data/run_iozone.yaml

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>