x86/hyperv: Add GUEST_IDLE_MSR support

Hyper-V may expose a HV_X64_MSR_GUEST_IDLE MSR via HYPERV_CPUID_FEATURES.

Reading this MSR triggers the host to transition the guest vCPU into an
idle state. This state can be exited via an IPI even if the read in the
guest happened from an interrupt disabled section.

Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Cc: chao.p.peng@intel.com
Cc: chao.gao@intel.com
Cc: isaku.yamahata@intel.com
Cc: tianyu.lan@microsoft.com
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Link: https://lkml.kernel.org/r/1538028104-114050-2-git-send-email-yi.y.sun@linux.intel.com