iotests: allow Valgrind checking all QEMU processes

iotests: allow Valgrind checking all QEMU processes

With the '-valgrind' option, let all the QEMU processes be run under
the Valgrind tool. The Valgrind own parameters may be set with its
environment variable VALGRIND_OPTS, e.g.
$ VALGRIND_OPTS="--leak-check=yes" ./check -valgrind <test#>
or they may be listed in the Valgrind checked file ./.valgrindrc or
~/.valgrindrc like
--memcheck:leak-check=no
--memcheck:track-origins=yes
To exclude a specific process from running under the Valgrind, the
corresponding environment variable VALGRIND_QEMU_<name> is to be set
to the empty string:
$ VALGRIND_QEMU_IO= ./check -valgrind <test#>
When QEMU-IO process is being killed, the shell report refers to the
text of the command in _qemu_io_wrapper(), which was modified with this
patch. So, the benchmark output for the tests 039, 061 and 137 is to be
changed also.

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>