NetworkPkg/SnpDxe: Shutdown SnpDxe on BeforeExitBootServices Event.

SnpDxe registers for an ExitBootServices callback and runs the
PXE_OPCODE_SHUTDOWN and PXE_OPCODE_STOP commands for any network
controllers that the driver is attached to.

Register the SnpDxe callback for gEfiEventBeforeExitBootServicesGuid
instead of gEfiEventExitBootServicesGuid to ensure the correct ordering:

1. ExitBootServices event
2. Network card shutdown sequence is completed

Also, close the event to prevent rerunning the shutdown if multiple
ExitBootServices events need to be called by the OS.

Signed-off-by: Aaron Pop <aaronpop@microsoft.com>

NetworkPkg/SnpDxe/Snp.c

@@ -32,6 +32,10 @@ SnpNotifyExitBootServices (
   //
   PxeShutdown (Snp);
   PxeStop (Snp);
+
+  // Since BeforeExitBootServices is run on each call, close event
+  // to prevent reentry.
+  gBS->CloseEvent (Event);
 }
 
 /**
@@ -655,7 +659,7 @@ SimpleNetworkDriverStart (
                     TPL_CALLBACK,
                     SnpNotifyExitBootServices,
                     Snp,
-                    &gEfiEventExitBootServicesGuid,
+                    &gEfiEventBeforeExitBootServicesGuid,
                     &Snp->ExitBootServicesEvent
                     );
     if (EFI_ERROR (Status)) {

NetworkPkg/SnpDxe/SnpDxe.inf

@@ -65,7 +65,7 @@
   NetLib
 
 [Guids]
-  gEfiEventExitBootServicesGuid                 ## SOMETIMES_CONSUMES ## Event
+  gEfiEventBeforeExitBootServicesGuid           ## SOMETIMES_CONSUMES ## Event
 
 [Protocols]
   gEfiSimpleNetworkProtocolGuid                 ## BY_START