Prevent from nonexistent paths in config.

In case of defining paths in config, which wrapper is trying to
normalize, sometimes it may happen, that paths can be either local or
remote - depending on the intention, i.e. one can add:

cdrom_drive_0: foo.iso

where foo can be either an unpacked file or just some companion image,
which exists within config file.

In this patch calculated path is checked against file existence, and if
it doesn't exists, original value is preserved.

fs_uae_wrapper/base.py

@@ -219,7 +219,11 @@ class Base(object):
                 changed_options[key] = abspath
                 continue
 
-            changed_options[key] = os.path.abspath(val)
+            _val = os.path.abspath(val)
+            if os.path.exists(_val):
+                changed_options[key] = _val
+            else:
+                changed_options[key] = val
 
         self.fsuae_options.update(changed_options)
 

fs_uae_wrapper/utils.py

@@ -144,6 +144,7 @@ def interpolate_variables(string, config_path, base=None):
         - $BASE
     """
 
+    _string = string
     if '$CONFIG' in string:
         conf_path = os.path.dirname(os.path.abspath(config_path))
         string = os.path.abspath(string.replace('$CONFIG', conf_path))
@@ -166,7 +167,9 @@ def interpolate_variables(string, config_path, base=None):
         if '$BASE' in string:
             string = string.replace('$BASE', base)
 
-    return string
+    if os.path.exists(string):
+        return string
+    return _string
 
 
 def get_config(conf_file):