KVM: x86/mmu: Restrict KVM_SW_PROTECTED_VM to the TDP MMU

KVM: x86/mmu: Restrict KVM_SW_PROTECTED_VM to the TDP MMU

Advertise and support software-protected VMs if and only if the TDP MMU is
enabled, i.e. disallow KVM_SW_PROTECTED_VM if TDP is enabled for KVM's
legacy/shadow MMU.  TDP support for the shadow MMU is maintenance-only,
e.g. support for TDX and SNP will also be restricted to the TDP MMU.

Fixes: 89ea60c2c7b5 ("KVM: x86: Add support for "protected VMs" that can utilize private memory")
Link: https://lore.kernel.org/r/20240222190612.2942589-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>