Use md5 as default hash method again

Older Debian releases are not able to cope with sha256 hashed
passwords and refuse login without any visible or accessible error
message.

This only affects the initially set root password and does not affect
passwords which are later set with the DomU's "passwd" tool.

bin/xen-create-image

@@ -1294,7 +1294,7 @@ sub setupDefaultOptions
     $CONFIG{ 'genpass' }     = 1;
     $CONFIG{ 'genpass_len' } = $default_genpass_len;
     $CONFIG{ 'password' }    = undef;
-    $CONFIG{ 'hash_method' } = 'sha256';
+    $CONFIG{ 'hash_method' } = 'md5';
 
     #
     #  The program to run to create a filesystem.