trace_usb_port_attach prints junk data

Running qemu with tracing (-D ~/qemu_trace -d trace:\*) will result in a trace file with unprintable characters.

example: usb_port_attach bus 0, port 1, devspeed <90>l<DB>.<D8>U, portspeed full+high

The problem is in hw/usb/bus.c usb_mask_to_str. If speedmask doesn't match any of the defined speed nothing is written to *dest and uninitialized data is printed to the log.

This happens with a real usb device that is forwarded into the machine.

My qemu version is 2.6.0 but it looks like the problem exists in latest git also.