iotests: add 'qemu' package location to PYTHONPATH in testenv

iotests: add 'qemu' package location to PYTHONPATH in testenv

We can drop the sys.path hacking in various places by doing
this. Additionally, by doing it in one place right up top, we can print
interesting warnings in case the environment does not look correct. (See
next commit.)

If we ever decide to change how the environment is crafted, all of the
"help me find my python packages" goop is all in one place, right in one
function.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210923180715.4168522-2-jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>