qemu-img: Set the ID of the block job in img_commit()

qemu-img: Set the ID of the block job in img_commit()

img_commit() creates a block job without an ID. This is no longer
allowed now that we require it to be unique and well-formed. We were
solving this by having a fallback in block_job_create(), but now that
we extended the API of commit_active_start() we can finally set an
explicit ID and revert that change.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>