usb: sanity check setup_index+setup_len in post_load

CVE-2013-4541

s->setup_len and s->setup_index are fed into usb_packet_copy as
size/offset into s->data_buf, it's possible for invalid state to exploit
this to load arbitrary data.

setup_len and setup_index should be checked to make sure
they are not negative.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
