Add argument to allow additional block-job options.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210116214705.822267-23-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>