Log all traffic of a specific usb device to a pcap file for later
inspection.  File format is compatible with linux usb monitor.

Usage:
  qemu -device usb-${somedevice},pcap=file.pcap
  wireshark file.pcap

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210119194452.2148048-1-kraxel@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>