Lower uhci timer freq when guest is idle.

UHCI emulation polls the device at a freq of 1000HZ, which consumes 25-30% CPU even though a Windows guest is completely idle.

This solution counts accumulated NAK packets. when it reaches to 64, then the guest is supposed to be idle and the freq is lowered to 10HZ.

This patch has been tested on Win7/WinXP/F17 guests.