Add ability to write files to be written on guest by providing filename.

box.py

@@ -304,6 +304,26 @@ class Config:
                 continue
             setattr(self, key, str(val))
 
+        # check if there are files to be written
+        if conf.get('boxpy_data', {}).get('write_files'):
+            for file_data in conf['boxpy_data']['write_files']:
+                fname = file_data.get('filename')
+                if not fname:
+                    print("WARNING: one of the entries in boxpy.write_files "
+                          "doesn't provide a filename.")
+                else:
+                    fname = os.path.expanduser(os.path.expandvars(fname))
+                    if not os.path.exists(fname):
+                        print(f"WARNING: file '{file_data['filename']}' "
+                              f"doesn't exists.")
+                    else:
+                        with open(fname) as fobj:
+                            file_data['content'] = fobj.read()
+                        del file_data['filename']
+                        if 'write_files' not in conf:
+                            conf['write_files'] = []
+                        conf['write_files'].append(file_data)
+
         # remove boxpy_data since it will be not needed on the guest side
         if conf.get('boxpy_data'):
             if conf['boxpy_data'].get('advanced'):