vmdk: Fix local_err in vmdk_create

In vmdk_create and vmdk_create_extent, initialize local_err before using
it, and don't leak it on error.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
