dm: fix fault Injection into VirtIO console backend

Add Null pointer check in init vq ring and add vq ring descriptor
 check in case cause Nullpointer exception.

Tracked-On: #5355
Signed-off-by: Liu Long <long.liu@intel.com>
Reviewed-by: Wang, Yu1 <yu1.wang@intel.com>

devicemodel/core/vmmapi.c

@@ -468,6 +468,7 @@ vm_map_gpa(struct vmctx *ctx, vm_paddr_t gaddr, size_t len)
 		}
 	}
 
+	pr_dbg("%s context memory is not valid!\n", __func__);
 	return NULL;
 }