qemu: get defaults from iotune group we move disk into

qemu: get defaults from iotune group we move disk into

For example if disk is not in the group and we want to move it
there then it makes sense to specify only the group name in API call.
Currently the destination group iotune settings will be overwritten
with the disk settings which I would say is not what one would expect.
Thus let's get defaults from the group we are moving to.

And if we are moving the brand new group then is makes sense to
copy the current disk iotune settings to the group.

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>