KVM: selftests: Verify non-postable IRQ remapping in IRQ test

Extend the eventfd IRQ test with an '-n' flag to route a subset of device
interrupts as NMIs (Non-Maskable Interrupts) into the guest using an
alternating pattern of 4 NMIs followed by 4 regular interrupts.

While this adds coverage for NMI injection, the primary goal is to
validate KVM's handling of non-postable interrupt delivery (AMD and Intel
IOMMUs only support posting fixed IRQs targeting a single vCPU).  KVM
has historically bungled handling transitions between posted and remapped
modes.  Use NMIs to stress the transitions, because they are a reliable,
architectural way to force these code paths.

Signed-off-by: David Matlack <dmatlack@google.com>
Co-developed-by: Josh Hilke <jrhilke@google.com>
Signed-off-by: Josh Hilke <jrhilke@google.com>
[sean: add GUEST_RECEIVED_INTERRUPT(), massage changelog]
Link: https://patch.msgid.link/20260626213534.3866178-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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