KVM: selftests: Use the right memslot for code, page-tables, and data allocations

Now that kvm_vm allows specifying different memslots for code, page tables,
and data, use the appropriate memslot when making allocations in
common/libraty code. Change them accordingly:

- code (allocated by lib/elf) use the CODE memslot
- stacks, exception tables, and other core data pages (like the TSS in x86)
  use the DATA memslot
- page tables and the PGD use the PT memslot
- test data (anything allocated with vm_vaddr_alloc()) uses the TEST_DATA
  memslot

No functional change intended. All allocators keep using memslot #0.

Cc: Sean Christopherson <seanjc@google.com>
Cc: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221017195834.2295901-10-ricarkol@google.com