cpuidle haltpoll driver and governor (v4)

The cpuidle-haltpoll driver with haltpoll governor allows the guest vcpus to poll for a specified amount of time before halting. This provides the following benefits to host side polling:

1) The POLL flag is set while polling is performed, which allows a remote vCPU to avoid sending an IPI (and the associated cost of handling the IPI) when performing a wakeup.

2) The VM-exit cost can be avoided.

The downside of guest side polling is that polling is performed even with other runnable tasks in the host.

Results comparing halt_poll_ns and server/client application where a small packet is ping-ponged:

host                                        --> 31.33
halt_poll_ns=300000 / no guest busy spin    --> 33.40   (93.8%)
halt_poll_ns=0 / guest_halt_poll_ns=300000  --> 32.73   (95.7%)

For the SAP HANA benchmarks (where idle_spin is a parameter of the previous version of the patch, results should be the same):

hpns == halt_poll_ns

                           idle_spin=0/   idle_spin=800/    idle_spin=0/
                           hpns=200000    hpns=0            hpns=800000
DeleteC06T03 (100 thread) 1.76           1.71 (-3%)        1.78   (+1%)
InsertC16T02 (100 thread) 2.14           2.07 (-3%)        2.18   (+1.8%)
DeleteC00T01 (1 thread)   1.34           1.28 (-4.5%)      1.29   (-3.7%)
UpdateC00T03 (1 thread)   4.72           4.18 (-12%)       4.53   (-5%)

V2:

- Move from x86 to generic code (Paolo/Christian)
- Add auto-tuning logic (Paolo)
- Add MSR to disable host side polling (Paolo)

V3:

- Do not be specific about HLT VM-exit in the documentation (Ankur Arora)
- Mark tuning parameters static and __read_mostly (Andrea Arcangeli)
- Add WARN_ON if host does not support poll control (Joao Martins)
- Use sched_clock and cleanup haltpoll_enter_idle (Peter Zijlstra)
- Mark certain functions in kvm.c as static (kernel test robot)
- Remove tracepoints as they use RCU from extended quiescent state (kernel test robot)

V4:
- Use a haltpoll governor, use poll_state.c poll code (Rafael J. Wysocki)