virttest.qemu_vm: Add freq option to usb device

USB EHCI controller has a 'freq' option, it's used to specify controller
Wakeup frequency.  Effect is that ehci will batch the frame processing
and process 4 frames per wakeup.  Reduces the cpu load, but also reduces
usb performance.  Not guest-visible.

The default value of this option is 1000, which equals the usb frame rate.
Can be reduced to -- for example -- 250.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>