lib: domain: Emphasise that users should wait for block job READY state via events

lib: domain: Emphasise that users should wait for block job READY state via events

The transition to the ready state is best observed by events as it's
ansynchronous and does not hint users to do polling. As currently only
the qemu driver supports block copy and block commit and the ready state
event was introduced by qemu 1.3 we can fully switch to the new
approach.

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