The method is not popular in iotests, we prefer use vm.qmp() and then
check success by hand. But that's not optimal. To simplify movement to
vm.cmd() let's support same interface improvements like in vm.qmp().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20231006154125.1068348-7-vsementsov@yandex-team.ru
Signed-off-by: John Snow <jsnow@redhat.com>