USB HID does not support Set_Idle

USB HID does not support Set_Idle

I found that the QEMU USB keyboard support does not work properly with
the Set_Idle command.  Once a non-zero value is given to Set_Idle,
then the keyboard reports an event on every poll - not based on the
time issued in the Set_Idle command.

I changed the code (see patch below) and it works for me.  I'm not
that familiar with the qemu internals, so I'm not sure if this is the
best way to implement this feature.

-Kevin

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>