qapi: handle visitor->type_size() in QapiDeallocVisitor

visit_type_size() requires either visitor->type_size() or
visitor_uint64() to be implemented, otherwise a NULL function pointer is
invoked.

It is possible to trigger this crash as follows:

  $ qemu-system-x86_64 -netdev tap,sndbuf=0,id=netdev0 \
                       -device virtio-blk-pci,netdev=netdev0

The 'sndbuf' option has type "size".

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
