KVM: selftests: Add test for race in kvm_recalculate_apic_map()

KVM: selftests: Add test for race in kvm_recalculate_apic_map()

Keep switching between LAPIC_MODE_X2APIC and LAPIC_MODE_DISABLED during
APIC map construction to hunt for TOCTOU bugs in KVM.  KVM's optimized map
recalc makes multiple passes over the list of vCPUs, and the calculations
ignore vCPU's whose APIC is hardware-disabled, i.e. there's a window where
toggling LAPIC_MODE_DISABLED is quite interesting.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230602233250.1014316-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>