KVM: selftests: Assert that eventfd() succeeds in Xen shinfo test

KVM: selftests: Assert that eventfd() succeeds in Xen shinfo test

Assert that eventfd() succeeds in the Xen shinfo test instead of skipping
the associated testcase.  While eventfd() is outside the scope of KVM, KVM
unconditionally selects EVENTFD, i.e. the syscall should always succeed.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>