Partially resolve last unknown BitLocker header field

This last field (offset 0x18) is a size, but I'm not sure yet of what
exactly. This may be a size only in a specific context as this field is
used (for what I've seen) only when curr_state is at 2.
Better handling of virtualized regions

What I call virtualized region here is a BitLocker area reported as
filled with zeroes when looked at from the NTFS layer.

A new virtualized region appears in W$ 8 encrypted volumes. I'm not sure
what it represents, but it's here. Therefore, change the way we're
dealing with these areas to improve it, hopefully.