Increase read chunk size to 1MB while uploading file

With a small chunk size (the previous chunk size was 8k), the server needs to
make many read/write operations which was consuming a lot of memory from
Kimchi server for larger files. So increase it to 1MB to do not overload
the server.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>