KVM: x86/xen: Add self tests for KVM_XEN_HVM_CONFIG_EVTCHN_SEND

Test a combination of event channel send, poll and timer operations.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20220303154127.202856-18-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>