hw/9pfs/virtio-9p-local.c: use snprintf() instead of sprintf()

'ctx->fs_root' + 'path'/'fullname.data' may be larger than PATH_MAX, so
need use snprintf() instead of sprintf() just like another area have done
in 9pfs. This could possibly result in the truncation of pathname, which we
address in the follow up patch.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
