KVM: selftests: Add guest_memfd tests for mmap and NUMA policy support

Add tests for NUMA memory policy binding and NUMA aware allocation in
guest_memfd. This extends the existing selftests by adding proper
validation for:
  - KVM GMEM set_policy and get_policy() vm_ops functionality using
    mbind() and get_mempolicy()
  - NUMA policy application before and after memory allocation

Run the NUMA mbind() test with and without INIT_SHARED, as KVM should allow
doing mbind(), madvise(), etc. on guest-private memory, e.g. so that
userspace can set NUMA policy for CoCo VMs.

Run the NUMA allocation test only for INIT_SHARED, i.e. if the host can't
fault-in memory (via direct access, madvise(), etc.) as move_pages()
returns -ENOENT if the page hasn't been faulted in (walks the host page
tables to find the associated folio)

[sean: don't skip entire test when running on non-NUMA system, test mbind()
       with private memory, provide more info in assert messages]

Signed-off-by: Shivank Garg <shivankg@amd.com>
Tested-by: Ashish Kalra <ashish.kalra@amd.com>
Link: https://lore.kernel.org/r/20251016172853.52451-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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