KVM: selftests: Setup vcpu_alias only for minor mode test

KVM: selftests: Setup vcpu_alias only for minor mode test

This fixes two things:

- Unbreaks MISSING mode test on anonymous memory type

- Prefault alias mem before uffd thread creations, otherwise the uffd
  thread timing will be inaccurate when guest mem size is large, because
  it'll take prefault time into total time.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20230427201112.2164776-2-peterx@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>