When c-state and mwait is passthroughed to the guest, enabling intel
idle driver can exploit them and benefit the performance greatly.
And potentially it can boost other features like haltpolling.

The user should remove idle=* from kernel cmdline to put it to work.
Signed-off-by: NYihao Wu <wuyihao@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>