util: xleep for Windows.

util: xleep for Windows.

Windows does not have a sleep(seconds). But it does have
a Sleep(milliseconds). Sleep() in windows does not have a
return value. Since we are not using the return value for xsleep()
anywhere as of now, don't return any.

Introduced by commit 275eebb9 (utils: Introduce xsleep for RCU quiescent state)

CC: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>