qcdm: fix QCDM packet decapsulation

Rename and document the encapsulate/decapsulate functions, CRC-check
the incoming packet, and make callers aware of the difference in how
big the decapsulated packet is versus how many bytes they should
discard from the buffer (since the decapsulated packet is at least
3 bytes shorter than the incoming packet due to the CRC + framing).

libqcdm/src/commands.c

@@ -61,7 +61,7 @@ qcdm_cmd_version_info_new (char *buf, gsize len, GError **error)
     memset (cmd, 0, sizeof (cmd));
     cmd->code = DIAG_CMD_VERSION_INFO;
 
-    return dm_prepare_buffer (cmdbuf, sizeof (*cmd), sizeof (cmdbuf), buf, len);
+    return dm_encapsulate_buffer (cmdbuf, sizeof (*cmd), sizeof (cmdbuf), buf, len);
 }
 
 QCDMResult *