It's done the exact same way as uploading a file
with action=upload,
except that the name of the parameter is 'xml', not 'file'. I've
updated the documentation for import [1] and upload [2] to mention
this.
[1]
http://www.mediawiki.org/wiki/API:Edit_-_Import
[2]
http://www.mediawiki.org/wiki/API:Edit_-_Uploading_files
Thanks, finally got it working! I've updated the Import page with an
actual code example.
Cheers,
-jani