KVM: selftests: Reuse virt mapping functions for nested EPTs

KVM: selftests: Reuse virt mapping functions for nested EPTs

Rework tdp_map() and friends to use __virt_pg_map() and drop the custom
EPT code in __tdp_pg_map() and tdp_create_pte().  The EPT code and
__virt_pg_map() are practically identical, the main differences are:
  - EPT uses the EPT struct overlay instead of the PTE masks.
  - EPT always assumes 4-level EPTs.

To reuse __virt_pg_map(), extend the PTE masks to work with EPT's RWX and
X-only capabilities, and provide a tdp_mmu_init() API so that EPT can pass
in the EPT PTE masks along with the root page level (which is currently
hardcoded to '4').

Don't reuse KVM's insane overloading of the USER bit for EPT_R as there's
no reason to multiplex bits in the selftests, e.g. selftests aren't trying
to shadow guest PTEs and thus don't care about funnelling protections into
a common permissions check.

Another benefit of reusing the code is having separate handling for
upper-level PTEs vs 4K PTEs, which avoids some quirks like setting the
large bit on a 4K PTE in the EPTs.

For all intents and purposes, no functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20251230230150.4150236-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>