hv: sched_bvt: add BVT scheduler

BVT (Borrowed virtual time) scheduler is used to schedule vCPUs on pCPU.
It has the concept of virtual time, vCPU with earliset virtual time is
dispatched first.

Main concepts:

tick timer:
    a period tick is used to measure the physcial time in units of MCU
    (minimum charing unit).
runqueue:
    thread in the runqueue is ordered by virtual time.
weight:
    each thread receives a share of the pCPU in proportion to its
    weight.
context switch allowance:
    the physcial time by which the current thread is allowed to advance
    beyond the next runnable thread.
warp:
    a thread with warp enabled will have a change to minus a value (Wi)
    from virtual time to achieve higher priority.
virtual time:
    AVT: actual virtual time, advance in proportional to weight.
    EVT: effective virtual time.
         EVT <- AVT - ( warp ? Wi : 0 )
    SVT: scheduler virtual time, the minimum AVT in the runqueue.

Tracked-On: #4410

Signed-off-by: Conghui Chen <conghui.chen@intel.com>
Signed-off-by: Shuo A Liu <shuo.a.liu@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/arch/x86/Kconfig

@@ -55,6 +55,15 @@ config SCHED_IORR
           IORR (IO sensitive Round Robin) scheduler supports multipule vCPUs running on
 	  on one pCPU, and they will be scheduled by a IO sensitive round robin policy.
 
+config SCHED_BVT
+        bool "BVT scheduler"
+        help
+	  BVT (Borrowed Virtual time) is virtual time based scheduling algorithm, it
+	  dispatching the runnable thread with the earliest effective virtual time.
+	  TODO: BVT scheduler will be built on top of prioritized scheduling mechanism,
+	  i.e. higher priority threads get scheduled first, and same priority tasks are
+	  scheduled per BVT.
+
 endchoice