There are two practical problems with bdrv_get_allocated_file_size()'s
default right now:
(1) For drivers with children, we should generally sum all their sizes
    instead of just passing the request through to bs->file.  The latter
    is good for filters, but not so much for format drivers.

(2) Filters need not have bs->file, so we should actually go to the
    filtered child instead of hard-coding bs->file.

Fix this by splitting the default implementation into three branches:
(1) For filter drivers: Return the size of the filtered child
(2) For protocol drivers: Return -ENOTSUP, because the default
    implementation cannot make a guess
(3) For other drivers: Sum all data-bearing children's sizes

Signed-off-by: Max Reitz <mreitz@redhat.com>