x86/paravirt: Use PVOP_* for paravirt calls

Doing unconditional indirect calls through the pv_ops vector is weird.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210624095148.437720419@infradead.org

arch/x86/include/asm/paravirt.h

@@ -52,11 +52,11 @@ void __init paravirt_set_cap(void);
 /* The paravirtualized I/O functions */
 static inline void slow_down_io(void)
 {
-	pv_ops.cpu.io_delay();
+	PVOP_VCALL0(cpu.io_delay);
 #ifdef REALLY_SLOW_IO
-	pv_ops.cpu.io_delay();
-	pv_ops.cpu.io_delay();
-	pv_ops.cpu.io_delay();
+	PVOP_VCALL0(cpu.io_delay);
+	PVOP_VCALL0(cpu.io_delay);
+	PVOP_VCALL0(cpu.io_delay);
 #endif
 }