fw_cfg_mem: add read memory region callback

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20180912160118.21158-2-liq3ea@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/nvram/fw_cfg.c

@@ -434,6 +434,11 @@ static bool fw_cfg_data_mem_valid(void *opaque, hwaddr addr,
     return addr == 0;
 }
 
+static uint64_t fw_cfg_ctl_mem_read(void *opaque, hwaddr addr, unsigned size)
+{
+    return 0;
+}
+
 static void fw_cfg_ctl_mem_write(void *opaque, hwaddr addr,
                                  uint64_t value, unsigned size)
 {
@@ -468,6 +473,7 @@ static bool fw_cfg_comb_valid(void *opaque, hwaddr addr,
 }
 
 static const MemoryRegionOps fw_cfg_ctl_mem_ops = {
+    .read = fw_cfg_ctl_mem_read,
     .write = fw_cfg_ctl_mem_write,
     .endianness = DEVICE_BIG_ENDIAN,
     .valid.accepts = fw_cfg_ctl_mem_valid,