KVM: selftests: Refactor nested_map() to specify target level

Refactor nested_map() to specify that it explicityl wants 4K mappings
(the existing behavior) and push the implementation down into
__nested_map(), which can be used in subsequent commits to create huge
page mappings.

No function change intended.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220520233249.3776001-5-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>