qemu: prepare blockjob complete event error usage

qemu: prepare blockjob complete event error usage

This patch pass event error up to the place where we can
use it. Error is passed only for sync blockjob event mode
as we can't use the error in async mode. In async mode we
just pass the event details to the client thru event API
but current blockjob event API can not carry extra parameter.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>