KVM: selftests: Add vm->memslots[] and enum kvm_mem_region_type

The vm_create() helpers are hardcoded to place most page types (code,
page-tables, stacks, etc) in the same memslot #0, and always backed with
anonymous 4K.  There are a couple of issues with that.  First, tests
willing to differ a bit, like placing page-tables in a different backing
source type must replicate much of what's already done by the vm_create()
functions.  Second, the hardcoded assumption of memslot #0 holding most
things is spread everywhere; this makes it very hard to change.

Fix the above issues by having selftests specify how they want memory to be
laid out. Start by changing ____vm_create() to not create memslot #0; a
test (to come) will specify all memslots used by the VM.  Then, add the
vm->memslots[] array to specify the right memslot for different memory
allocators, e.g.,: lib/elf should use the vm->[MEM_REGION_CODE] memslot.
This will be used as a way to specify the page-tables memslots (to be
backed by huge pages for example).

There is no functional change intended. The current commit lays out memory
exactly as before. A future commit will change the allocators to get the
region they should be using, e.g.,: like the page table allocators using
the pt memslot.

Cc: Sean Christopherson <seanjc@google.com>
Cc: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221017195834.2295901-8-ricarkol@google.com