Release formats preferences aren't respected when using Acoustid matching

I did the test with untagged files matching http://musicbrainz.org/ws/2/release?release-group=8babed0d-81c1-4541-ab03-582fc9a31482&limit=100&inc=media+labels

Set Options > Metadata > Preferred Releases > Preferred Release Formats to:

• Digital Media
• Vinyl

Load files, cluster them, Scan (acoustid matching), release is found, but it defaults on vinyl format instead of digital media.

I narrowed down the issue to acoustid.parse_recording() which do not fill 'format' in the rebuilt xml node. Applying the attached 2-lines patch seems to fix this issue.