conf: expand iotune params if only group name is given

conf: expand iotune params if only group name is given

Currently, if only iotune group name is given for some disk and
no any params then later start of domain will fail. I guess it
will be convenient to allow such configuration if there is
another disk in the same iotune group with iotune params set. The
meaning is that the first disk have same iotunes and the latter.
Thus one can easily add a disk to iotune group - just add group
name parameter and no need to copy all the params.

Also let's expand iotunes params in the described case so we don't
need to refer to another disk to know iotunes and this will make
logic in many places simple.

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>