memory: Return -1 again on reads from unsigned regions

This restore the behavior prior to b018ddf6 which accidentally changed
the return code to 0. Specifically guests probing for register existence
were affected by this.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

memory.c

@@ -840,7 +840,7 @@ static uint64_t unassigned_mem_read(void *opaque, hwaddr addr,
     if (current_cpu != NULL) {
         cpu_unassigned_access(current_cpu, addr, false, false, 0, size);
     }
-    return 0;
+    return -1ULL;
 }
 
 static void unassigned_mem_write(void *opaque, hwaddr addr,