hw/usb: avoid format truncation warning when formatting port name

hw/usb: avoid format truncation warning when formatting port name

hw/usb/hcd-xhci.c: In function ‘usb_xhci_realize’:
hw/usb/hcd-xhci.c:3339:66: warning: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 5 [-Wformat-trunca\
tion=]
 3339 |             snprintf(port->name, sizeof(port->name), "usb2 port #%d", i+1);
      |                                                                  ^~
hw/usb/hcd-xhci.c:3339:54: note: directive argument in the range [1, 2147483647]
 3339 |             snprintf(port->name, sizeof(port->name), "usb2 port #%d", i+1);
      |                                                      ^~~~~~~~~~~~~~~

The xhci code formats the port name into a fixed length
buffer which is only large enough to hold port numbers
upto 5 digits in decimal representation. We're never
going to have a port number that large, so aserting the
port number is sensible is sufficient to tell GCC the
formatted string won't be truncated.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190412121626.19829-5-berrange@redhat.com>

[ kraxel: also s/int/unsigned int/ to tell gcc they can't
          go negative. ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>