AES XTS: supporting custom iv from openssl enc command

openssl enc command with -aes-xxx-xts doesnt work if an IV is specified
as below:
openssl enc -engine af_alg -aes-256-xts -in <plaintext_file> -out
<output_encrypted_file> -K
0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef -iv
00000000000000000000000000000000
I am proposing a minor enhancement in EVP_CipherInit_ex() to include
case EVP_CIPH_XTS_MODE which currently is not present.