hv_tlbflush allows the guest OS to trigger tlb shootdowns via a
hypercall. This allows CPUs to be identified via their vpindex (which
makes hv_vpindex a prerequisite to hv_tlbflush, but that is already
handled in our code). In overcommited configurations, where multiple
vCPUs reside on one pCPU, this increases performance of guest tlb
flushes, by only flushing each pCPU once. It also allows multiple tlb
flushes with only one vmexit.
