Improve default device detection

[fysnet]

This improves the detection of the default speed of a device when given in bochsrc.

It will first get the max speed of the port given. For example, uhci and ohci will return FULL, ehci will return HIGH.
xHCI will return SUPER if the port is a super-speed port, else it will return HIGH.

Then it will compare this with the max speed allowed for that device, returning the highest common speed.

Of course, if the speed option is given, the above is overwritten by the specified speed.

Both checks are still performed on the result.

1. does device allow the result speed?
2. does the controller allow the result speed?