python: use vm.cmd() instead of vm.qmp() where appropriate

python: use vm.cmd() instead of vm.qmp() where appropriate

In many cases we just want an effect of qmp command and want to raise
on failure.  Use vm.cmd() method which does exactly this.

The commit is generated by command

  git grep -l '\.qmp(' | xargs ./scripts/python_qmp_updater.py

And then, fix self.assertRaises to expect ExecuteError exception in
tests/qemu-iotests/124

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20231006154125.1068348-16-vsementsov@yandex-team.ru
Signed-off-by: John Snow <jsnow@redhat.com>