vmdk: convert error code to use errp

Convert "fprintf(stderr,..." and standardize error messages:

Remove a few local_error's and use errp.

Remove "VMDK:" or "Vmdk:" prefixes in error message and fix to upper
case.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>