We can properly track the job type when starting the job so that we
don't have to infer it later.

This patch also adds an enum of block job types specific to qemu
(qemuBlockjobType) which mirrors the public block job types
(virDomainBlockJobType) but allows for other types to be added later
which will not be public.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>