KVM: x86/mmu: Expose number of shadow MMU shadow pages as a stat

Turn arch.n_used_mmu_pages into a stat, mmu_shadow_pages, as the number of
live shadow pages is arguably _the_ most critical datapoint when it comes
to analyzing the shadow MMU.  Before the TDP MMU came along, i.e. when the
shadow MMU was the only MMU, explicitly tracking the number of shadow pages
wasn't as interesting, because the same information could more or less be
gleaned from the pages_{1g,2m,4k} stats.  But with the TDP MMU, where the
shadow MMU is only used for nested TDP, it becomes extremely difficult, if
not impossible, to determine which SPTEs are coming from the TDP MMU, and
which are coming from the shadow MMU.

E.g. when triaging/debugging shadow MMU performance issues due to "too many
shadow pages", being able to observe that 99%+ of all shadow pages are
unsync is critical to being able to deduce that KVM is effectively leaking
shadow pages.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260612133727.411902-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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