usb-redir: fix stack overflow in usbredir_log_data

Don't reinvent a broken wheel, just use the hexdump function we have.

Impact: low, broken code doesn't run unless you have debug logging
enabled.

Reported-by: 李强 <liqiang6-s@360.cn>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170509110128.27261-1-kraxel@redhat.com
