qemu: fix using defaults when setting persistent iotune params

qemu: fix using defaults when setting persistent iotune params

virDomainSetBlockIoTune not simply sets the iotune params given in API
but use current settings for all the omitted params. Unfortunately
it uses current settings for active config when setting inactive
params. Let's fix it.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>