lisa.trace: Ensure meta event source df use bytes

Meta events (like userspace@... or trace_printk@...) are extracted from existing dataframes. The text data is expected to be encoded as bytes, so that parser that can provide bytes out of the box avoid a conversion (and x4 in memory consumption incurred by str).

That said, some parsers may return string columns, in which case it needs to be converted, as regexes are written against bytes type.