block/vmdk: add basic .bdrv_check support

this adds a basic vmdk corruption check. it should detect severe
table corruptions and file truncation.

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