Internal routines in vmdk.c previously return -1 on error and 0 on
success. More return values are useful for future changes such as
zeroed-grain GTE. Change all the magic `return 0` and `return -1` to
macro names:
* VMDK_OK 0
* VMDK_ERROR (-1)
* VMDK_UNALLOC (-2)
* VMDK_ZEROED (-3)
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>