aes: xts: Rename iv to data_unit

XTS doesn't have an IV, it has a "Data Unit". Rename iv for parity with the
XTS standard.
diff --git a/tests/suites/test_suite_aes.function b/tests/suites/test_suite_aes.function
index d781d2e..165f702 100644
--- a/tests/suites/test_suite_aes.function
+++ b/tests/suites/test_suite_aes.function
@@ -152,12 +152,12 @@
 /* END_CASE */
 
 /* BEGIN_CASE depends_on:MBEDTLS_CIPHER_MODE_XTS */
-void aes_encrypt_xts( char *hex_key_string, char *hex_iv_string,
+void aes_encrypt_xts( char *hex_key_string, char *hex_data_unit_string,
                       char *hex_src_string, char *hex_dst_string,
                       int data_unit_len, int xts_result )
 {
     unsigned char key_str[100] = { 0, };
-    unsigned char iv_str[100]  = { 0, };
+    unsigned char data_unit_str[100] = { 0, };
     unsigned char src_str[100] = { 0, };
     unsigned char dst_str[100] = { 0, };
     unsigned char output[100]  = { 0, };
@@ -167,13 +167,13 @@
     mbedtls_aes_xts_init( &ctx );
 
     key_len = unhexify( key_str, hex_key_string );
-    unhexify( iv_str, hex_iv_string );
+    unhexify( data_unit_str, hex_data_unit_string );
     data_len = unhexify( src_str, hex_src_string );
     TEST_ASSERT( data_len == data_unit_len / 8 );
 
     mbedtls_aes_xts_setkey_enc( &ctx, key_str, key_len * 8 );
 
-    TEST_ASSERT( mbedtls_aes_crypt_xts( &ctx, MBEDTLS_AES_ENCRYPT, data_len, iv_str, src_str, output ) == xts_result );
+    TEST_ASSERT( mbedtls_aes_crypt_xts( &ctx, MBEDTLS_AES_ENCRYPT, data_len, data_unit_str, src_str, output ) == xts_result );
     if( xts_result == 0 )
     {
         hexify( dst_str, output, data_len );
@@ -187,12 +187,12 @@
 /* END_CASE */
 
 /* BEGIN_CASE depends_on:MBEDTLS_CIPHER_MODE_XTS */
-void aes_decrypt_xts( char *hex_key_string, char *hex_iv_string,
+void aes_decrypt_xts( char *hex_key_string, char *hex_data_unit_string,
                       char *hex_src_string, char *hex_dst_string,
                       int data_unit_len, int xts_result )
 {
     unsigned char key_str[100] = { 0, };
-    unsigned char iv_str[100]  = { 0, };
+    unsigned char data_unit_str[100] = { 0, };
     unsigned char src_str[100] = { 0, };
     unsigned char dst_str[100] = { 0, };
     unsigned char output[100]  = { 0, };
@@ -202,13 +202,13 @@
     mbedtls_aes_xts_init( &ctx );
 
     key_len = unhexify( key_str, hex_key_string );
-    unhexify( iv_str, hex_iv_string );
+    unhexify( data_unit_str, hex_data_unit_string );
     data_len = unhexify( src_str, hex_src_string );
     TEST_ASSERT( data_len == data_unit_len / 8 );
 
     mbedtls_aes_xts_setkey_dec( &ctx, key_str, key_len * 8 );
 
-    TEST_ASSERT( mbedtls_aes_crypt_xts( &ctx, MBEDTLS_AES_DECRYPT, data_len, iv_str, src_str, output ) == xts_result );
+    TEST_ASSERT( mbedtls_aes_crypt_xts( &ctx, MBEDTLS_AES_DECRYPT, data_len, data_unit_str, src_str, output ) == xts_result );
     if( xts_result == 0 )
     {
         hexify( dst_str, output, data_len );