KVM: selftests: Add CONFIG_EVENTFD for irqfd selftest

KVM: selftests: Add CONFIG_EVENTFD for irqfd selftest

In 7e9b231c402a ("KVM: selftests: Add a KVM_IRQFD test to verify
uniqueness requirements") we added a test for the newly added irqfd
support but since this feature works with eventfds it won't work unless
the kernel has been built wth eventfd support.  Add CONFIG_EVENTFD to
the list of required options for the KVM selftests.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250710-kvm-selftests-eventfd-config-v1-1-78c276e4b80f@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>