vmdk: Clean up control flow in vmdk_parse_extents() a bit

Factor out loop stepping to turn a while-loop with goto into a
for-loop with continue.

Cc: Fam Zheng <famz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-Id: <1450452927-8346-18-git-send-email-armbru@redhat.com>
