hyperv: process SIGNAL_EVENT hypercall

hyperv: process SIGNAL_EVENT hypercall

Add handling of SIGNAL_EVENT hypercall.  For that, provide an interface
to associate an EventNotifier with an event connection number, so that
it's signaled when the SIGNAL_EVENT hypercall with the matching
connection ID is called by the guest.

Support for using KVM functionality for this will be added in a followup
patch.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20180921082217.29481-8-rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>