avocado_qemu/__init__.py: factor out the qemu-img finding

avocado_qemu/__init__.py: factor out the qemu-img finding

Factor out the code that finds the qemu-img binary in the
QemuSystemTest class and create a new get_qemu_img() function
with it. This function will get called also from the new code
in tuxrun_baselines.py avocado test-case.

Signed-off-by: Kautuk Consul <kconsul@linux.vnet.ibm.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Suggested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230421042322.684093-2-kconsul@linux.vnet.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230424092249.58552-12-alex.bennee@linaro.org>