bdrv_commit() could return 0 or 1 on success, depending on whether or
not the last sector was allocated in the overlay and whether the overlay
format had a .bdrv_make_empty callback.

Most callers ignored it, but qemu-img commit would print an error
message while the operation actually succeeded.

Also clean up the handling of I/O errors to return the real error code
instead of -EIO.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>