KVM: x86/mmu: Further check old SPTE is leaf for spurious prefetch fault

Instead of simply treating a prefetch fault as spurious when there's a
shadow-present old SPTE, further check if the old SPTE is leaf to determine
if a prefetch fault is spurious.

It's not reasonable to treat a prefetch fault as spurious when there's a
shadow-present non-leaf SPTE without a corresponding shadow-present leaf
SPTE. e.g., in the following sequence, a prefetch fault should not be
considered spurious:
1. add a memslot with size 4K
2. prefault GPA A in the memslot
3. delete the memslot (zap all disabled)
4. re-add the memslot with size 2M
5. prefault GPA A again.
In step 5, the prefetch fault attempts to install a 2M huge entry.
Since step 3 zaps the leaf SPTE for GPA A while keeping the non-leaf SPTE,
the leaf entry will remain empty after step 5 if the fetch fault is
regarded as spurious due to a shadow-present non-leaf SPTE.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20250318013111.5648-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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