block: Use block_job_get() in find_block_job()

find_block_job() looks for a block backend with a specified name,
checks whether it has a block job and acquires its AioContext.

We want to identify jobs by their ID and not by the block backend
they're attached to, so this patch ignores the backends altogether and
gets the job directly. Apart from making the code simpler, this will
allow us to find block jobs once they start having user-specified IDs.

To ensure backward compatibility we keep ERROR_CLASS_DEVICE_NOT_ACTIVE
as the error class if the job doesn't exist. In subsequent patches
we'll also need to keep the device name as the default job ID if the
user doesn't specify a different one.

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>
