dm:use acrn-dm logger function instread of printf

Use acrn-dm logger function instread of printf,
this helps the stability testing log capture.

Tracked-On: #4098
Signed-off-by: Mingqiang Chi <mingqiang.chi@intel.com>
Reviewed-by: Cao Minggui <minggui.cao@intel.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>

devicemodel/core/main.c

@@ -695,7 +695,7 @@ vm_loop(struct vmctx *ctx)
 			vm_suspend_resume(ctx);
 		}
 	}
-	printf("VM loop exit\n");
+	pr_err("VM loop exit\n");
 }
 
 static int