KVM: selftests: Add EVTCHNOP_send slow path test to xen_shinfo_test

KVM: selftests: Add EVTCHNOP_send slow path test to xen_shinfo_test

When kvm_xen_evtchn_send() takes the slow path because the shinfo GPC
needs to be revalidated, it used to violate the SRCU vs. kvm->lock
locking rules and potentially cause a deadlock.

Now that lockdep is learning to catch such things, make sure that code
path is exercised by the selftest.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>