qemu: process: refresh block jobs on reconnect

qemu: process: refresh block jobs on reconnect

Block job state was widely untracked by libvirt across restarts which
was allowed by a stateless block job finishing handler which discarded
disk state and redetected it. This is undesirable since we'll need to
track more information for individual blockjobs due to -blockdev
integration requirements.

In case of legacy blockjobs we can recover whether the job is present at
reconnect time by querying qemu. Adding tracking whether a job is
present will allow simplification of the non-shared-storage cancellation
code.

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