qemu-iotests: extend the check script to prepare supporting valgrind for python tests

qemu-iotests: extend the check script to prepare supporting valgrind for python tests

Currently, the check script only parses the option and sets the
VALGRIND_QEMU environmental variable to "y".
Add another local python variable that prepares the command line,
identical to the one provided in the test scripts.

Because the python script does not know in advance the valgrind
PID to assign to the log file name, use the "%p" flag in valgrind
log file name that automatically puts the process PID at runtime.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210809090114.64834-11-eesposit@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>