Implement the virtio block 'get-ident' operation. This eliminates the
annoying verbose boot error of the form

   g_handleattr: vtbd0 bio_length 24 len 28 -> EFAULT

The ident returned by bhyve is a text string 'BHYVE-XXXX-XXXX', where
the X's are the first bytes of the md5 hash of the backing filename.