KVM: x86: Add hook for determining max NPT mapping level

In the case of SEV-SNP, whether or not a 2MB page can be mapped via a
2MB mapping in the guest's nested page table depends on whether or not
any subpages within the range have already been initialized as private
in the RMP table. The existing mixed-attribute tracking in KVM is
insufficient here, for instance:

- gmem allocates 2MB page
- guest issues PVALIDATE on 2MB page
- guest later converts a subpage to shared
- SNP host code issues PSMASH to split 2MB RMP mapping to 4K
- KVM MMU splits NPT mapping to 4K
- guest later converts that shared page back to private

At this point there are no mixed attributes, and KVM would normally
allow for 2MB NPT mappings again, but this is actually not allowed
because the RMP table mappings are 4K and cannot be promoted on the
hypervisor side, so the NPT mappings must still be limited to 4K to
match this.

Add a hook to determine the max NPT mapping size in situations like
this.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-ID: <20240501085210.2213060-3-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>