blockjob: Remove BlockJob.bs

There is a single remaining user in qemu-img, and another one in a test
case, both of which can be trivially converted to using BlockJob.blk
instead.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>