Kernel/Interrupts: Return boolean on whether we handled the interrupt

If we are in a shared interrupt handler, the called handlers might
indicate it was not their interrupt, so we should not increment the
call counter of these handlers.

Kernel/Interrupts/UnhandledInterruptHandler.h

@@ -17,7 +17,7 @@ public:
     explicit UnhandledInterruptHandler(u8 interrupt_vector);
     virtual ~UnhandledInterruptHandler();
 
-    virtual void handle_interrupt(const RegisterState&) override;
+    virtual bool handle_interrupt(const RegisterState&) override;
 
     [[noreturn]] virtual bool eoi() override;