KVM: x86/xen: intercept EVTCHNOP_send from guests

KVM: x86/xen: intercept EVTCHNOP_send from guests

Userspace registers a sending @port to either deliver to an @eventfd
or directly back to a local event channel port.

After binding events the guest or host may wish to bind those
events to a particular vcpu. This is usually done for unbound
and and interdomain events. Update requests are handled via the
KVM_XEN_EVTCHN_UPDATE flag.

Unregistered ports are handled by the emulator.

Co-developed-by: Ankur Arora <[email protected]>
Co-developed-By: David Woodhouse <[email protected]>
Signed-off-by: Joao Martins <[email protected]>
Signed-off-by: Ankur Arora <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20220303154127[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>