KVM: selftests: Test TPR / CR8 sync and interrupt masking

KVM: selftests: Test TPR / CR8 sync and interrupt masking

Add a few extra TPR / CR8 tests to x86's xapic_state_test to see if:
  * TPR is 0 on reset,
  * TPR, PPR and CR8 are equal inside the guest,
  * TPR and CR8 read equal by the host after a VMExit
  * TPR borderline values set by the host correctly mask interrupts in the
    guest.

These hopefully will catch the most obvious cases of improper TPR sync or
interrupt masking.

Do these tests both in x2APIC and xAPIC modes.
The x2APIC mode uses SELF_IPI register to trigger interrupts to give it a
bit of exercise too.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Acked-by: Naveen N Rao (AMD) <naveen@kernel.org>
[sean: put code in separate test]
Link: https://patch.msgid.link/20251205224937.428122-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>