virdomainjob: make drivers use job object in the domain object

virdomainjob: make drivers use job object in the domain object

This patch uses the job object directly in the domain object and
removes the job object from private data of all drivers that use
it as well as other relevant code (initializing and freeing the
structure).

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>