x86: Rationalize names of paging structures

Changes the name of paging structure entries to more clearly indicate the kind of object
they map to. Generally this is changing a `pde_small` to `pde_pt` to indicate that this
mapping refers to a page table, removing an inconsistency where `pde_large` indicated that
the mapping was for a large page. For the same reason the `ept_pde_4k` type is changed to
`ept_pde_pt` type to reflect what is present in the actual entry.

`pde_large` is left as 'large' and not explicitly given a size as code common between
ia32 and x86-64 manipulates these entities and 'large' is already a used abstraction
over the two potential page sizes so there is need to introduce a formal abstraction layer
and make the names in the structures more specific.

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

[STYLE_FIX]

SELFOUR-287: 32-bit vt-x implementation

This is an implementation of vt-x for x86 kernels running in
ia32 mode.