qemu: Remove stale comment outlining how to extend qemuDomainBlockPivot

qemu: Remove stale comment outlining how to extend qemuDomainBlockPivot

With -blockdev:

- we track the job and check it after restart
- have the ability to ask qemu to persist it to collect result
- have the ability to report errors.

This solves all points the comment outlined so remove it. Also all jobs
handle the disk state modification along with the event so there's
nothing special the comment says.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>