KVM: selftests: Verify APIC_ID is set when forcing x2APIC=>xAPIC transition

Add a sub-test to verify that KVM stuffs the APIC_ID when userspace forces
a transition from x2APIC to xAPIC without first disabling the APIC.  Such
a transition is architecturally disallowed (WRMSR will #GP), but needs to
be handled by KVM to allow userspace to emulate RESET (ignoring that
userspace should also stuff local APIC state on RESET).

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Link: https://lore.kernel.org/r/20230109130605.2013555-3-eesposit@redhat.com


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