KVM: selftests: Add infrastructure for getting vCPU binary stats

Now that the binary stats cache infrastructure is largely scope agnostic,
add support for vCPU-scoped stats.  Like VM stats, open and cache the
stats FD when the vCPU is created so that it's guaranteed to be valid when
vcpu_get_stats() is invoked.

Account for the extra per-vCPU file descriptor in kvm_set_files_rlimit(),
so that tests that create large VMs don't run afoul of resource limits.

To sanity check that the infrastructure actually works, and to get a bit
of bonus coverage, add an assert in x86's xapic_ipi_test to verify that
the number of HLTs executed by the test matches the number of HLT exits
observed by KVM.

Tested-by: Manali Shukla <Manali.Shukla@amd.com>
Link: https://lore.kernel.org/r/20250111005049.1247555-9-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>