This improves vmdk_create to use bdrv_* functions to replace qemu_open
and other fd functions. The error handling are improved as well. One
difference is that bdrv_pwrite will round up buffer to sectors, so for
description file, an extra bdrv_truncate is used in the end to drop
inding zeros.

Notes:

 - A bonus bug fix is correct endian is used in initializing GD entries.

 - ROUND_UP and DIV_ROUND_UP are used where possible.

I tested that new code produces exactly the same file as previously.

Signed-off-by: Fam Zheng <famz@redhat.com>
Tested-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>