OvmfPkg/VirtioNetDxe: fix SignalEvent() call

The SignalEvent() boot service takes an EFI_EVENT, not an (EFI_EVENT*).
Fix the call in the notification function of
"EFI_SIMPLE_NETWORK_PROTOCOL.WaitForPacket".

This is an actual bug. The reason it's never been triggered is likely that
the "SNP.WaitForPacket" event is rarely waited for by applications -- edk2
itself has zero instances of that, for example.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

OvmfPkg/VirtioNetDxe/Events.c

@@ -58,7 +58,7 @@ VirtioNetIsPacketAvailable (
   MemoryFence ();
 
   if (Dev->RxLastUsed != RxCurUsed) {
-    gBS->SignalEvent (&Dev->Snp.WaitForPacket);
+    gBS->SignalEvent (Dev->Snp.WaitForPacket);
   }
 }