arch-x86,dev: Make the I8259::getVector method protected.

Now that the I8259's vector is reported using a special memory access,
the getVector method doesn't need to be accessible outside of the class.
It's still useful internally though, since it nicely encapsulates what
should happen when an INTA signal is received.

Change-Id: I7da7c1f18fac97ffc62c965978f53fb4c5430de3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55698
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/x86/i8259.hh

@@ -83,6 +83,8 @@ class I8259 : public BasicPioDevice
     void requestInterrupt(int line);
     void handleEOI(int line);
 
+    int getVector();
+
   public:
     using Params = I8259Params;
 
@@ -119,7 +121,6 @@ class I8259 : public BasicPioDevice
     void signalInterrupt(int line);
     void raiseInterruptPin(int number);
     void lowerInterruptPin(int number);
-    int getVector();
 
     void serialize(CheckpointOut &cp) const override;
     void unserialize(CheckpointIn &cp) override;