Function read_used 

pub fn read_used(
    mem: &GuestMemory,
    used_addr: u64,
    queue_size: u16,
    used_idx: &mut u16,
) -> Option<(u16, u32)>

Read the next used ring entry, returning (desc_id, bytes_written) or None if no new entries are available.

Advances *used_idx when an entry is consumed.