hw: debugexit: add read callback

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20180912160118.21158-3-liq3ea@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/misc/debugexit.c

@@ -23,6 +23,11 @@ typedef struct ISADebugExitState {
     MemoryRegion io;
 } ISADebugExitState;
 
+static uint64_t debug_exit_read(void *opaque, hwaddr addr, unsigned size)
+{
+    return 0;
+}
+
 static void debug_exit_write(void *opaque, hwaddr addr, uint64_t val,
                              unsigned width)
 {
@@ -30,6 +35,7 @@ static void debug_exit_write(void *opaque, hwaddr addr, uint64_t val,
 }
 
 static const MemoryRegionOps debug_exit_ops = {
+    .read = debug_exit_read,
     .write = debug_exit_write,
     .valid.min_access_size = 1,
     .valid.max_access_size = 4,