KVM: selftests: Extend x86's sync_regs_test to check for event vector races

KVM: selftests: Extend x86's sync_regs_test to check for event vector races

Attempt to modify the to-be-injected exception vector to an illegal value
_after_ the sanity checks performed by KVM_CAP_SYNC_REGS's
arch/x86/kvm/x86.c:kvm_vcpu_ioctl_x86_set_vcpu_events().  Buggy KVM
versions will eventually yells loudly about attempting to inject a bogus
vector, e.g.

  WARNING: CPU: 0 PID: 1107 at arch/x86/kvm/x86.c:547 kvm_check_and_inject_events+0x4a0/0x500 [kvm]

  arch/x86/kvm/x86.c:exception_type():
  WARN_ON(vector > 31 || vector == NMI_VECTOR)

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230728001606.2275586-3-mhal@rbox.co
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>