'stream-start' has a parameter called 'backing-file', which is the
string to be written to bs->backing when the job finishes.

In the stream_start() implementation it is called 'backing_file_str',
but it the prototype in the header file it is called 'base_id'.

This patch fixes it so the name is the same in both cases and is
consistent with other cases (like commit_start()).
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>