Using the kernel's preferred types eliminates a source of friction for many
contributors, as the majority of KVM selftests contributions come from kernel
developers. The kernel names are also shorter, which allows for more concise
code, and in any many cases eliminates newlines thanks to shorter types and
parameter names.
Rename variables and parameters as well as types, e.g. gpa instead of paddr,
to again align with the kernel, and in a few cases to remove ambiguity, e.g.
where paddr is used to refer to a _host_ physical address.