qemu: fix crash in qemuDomainSetBlkioParameters without cgroups

qemu: fix crash in qemuDomainSetBlkioParameters without cgroups

If we don't have cgroups available and user tries to update blkio
parameters for running VM it will crash.

It should have been protected by the virCgroupHasController() check but
it was never called if the API was executed without any flags.

We call virDomainObjGetDefs() which sets `def` and `persistentDef` based
on the flags and these two variables should be used to figure out if we
need to update LIVE, CONFIG or both states.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1808293

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>