iotests/303: use dot slash for qcow2.py running

iotests/303: use dot slash for qcow2.py running

If you run './check 303', check includes common.config which adjusts
$PATH to include '.' first, and therefore finds qcow2.py on PATH.  But
if you run './303' directly, there is nothing to adjust PATH, and if
'.' is not already on your PATH by other means, the test fails because
the executable is not found.  Adjust how we invoke the helper
executable to avoid needing a PATH search in the first place.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210116134424.82867-3-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>