qemu: check iotune params same for all disk in group

qemu: check iotune params same for all disk in group

Currently it is possible to start a domain which have disks
in same iotune group and at the same time having different iotune
params. Both params set are passed to qemu in command line and the one
that is passed later down command line is get actually set.
Let's prohibit such configurations.

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>