hw/9pfs: Fix segfault with 9p2000.u

When guest tries to chmod a block or char device file over 9pfs,
the qemu process segfaults. With 9p2000.u protocol we use wstat to
change mode bits and client don't send extension information for
chmod. We need to check for size field to check whether extension
info is present or not.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Acked-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>