Kill all the macros for reading/writing image parts. New API looks like

* write_img_buf/write_img
  Write an object into an image. Reports 0 for OK, -1 for error. The _buf
  version accepts object size as an argument, the other one uses sizeof()

* read_img_buf/read_img
  Reads an object from image. Reports 0 for OK, -1 for error or EOF.

* read_img_buf_eof/read_img
  Reads an object from image. Reports 1 for OK, 0 for EOF and -1 for error.
  This is not symmetrical with the previous one, but it was done deliberately
  to make it possible to write code like

  ret = read_img_bug_eof();
  if (ret <= 0)
	return ret; /* 0 means OK, all is done, -1 means error was met */.

  ... /* 1 means object was read, can proceed */
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>