KVM: selftests: Add a KVM_IRQFD test to verify uniqueness requirements

Add a selftest to verify that eventfd+irqfd bindings are globally unique,
i.e. that KVM doesn't allow multiple irqfds to bind to a single eventfd,
even across VMs.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-14-seanjc@google.com


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