qcow2: Validate snapshot table offset/size (CVE-2014-0144)

This avoid unbounded memory allocation and fixes a potential buffer
overflow on 32 bit hosts.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
