dm: Fix some issues from string operations

The patch fix some string operations issues and also improve readability
of several snippet.

Tracked-On: #2133
Signed-off-by: Shuo A Liu <shuo.a.liu@intel.com>
Reviewed-by: Yonghua Huang <yonghua.huang@intel.com>

devicemodel/include/dm.h

@@ -32,6 +32,7 @@
 #include <stdbool.h>
 #include "types.h"
 #include "vmm.h"
+#include "dm_string.h"
 
 struct vmctx;
 extern int guest_ncpus;