memory: Provide separate handling of unassigned io ports accesses

Accesses to unassigned io ports shall return -1 on read and be ignored
on write. Ensure these properties via dedicated ops, decoupling us from
the memory core's handling of unassigned accesses.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

ioport.c

@@ -44,6 +44,22 @@ typedef struct MemoryRegionPortioList {
     MemoryRegionPortio ports[];
 } MemoryRegionPortioList;
 
+static uint64_t unassigned_io_read(void *opaque, hwaddr addr, unsigned size)
+{
+    return -1ULL;
+}
+
+static void unassigned_io_write(void *opaque, hwaddr addr, uint64_t val,
+                                unsigned size)
+{
+}
+
+const MemoryRegionOps unassigned_io_ops = {
+    .read = unassigned_io_read,
+    .write = unassigned_io_write,
+    .endianness = DEVICE_NATIVE_ENDIAN,
+};
+
 void cpu_outb(pio_addr_t addr, uint8_t val)
 {
     LOG_IOPORT("outb: %04"FMT_pioaddr" %02"PRIx8"\n", addr, val);