KVM: x86/mmu: Add tracepoint for splitting huge pages

Add a tracepoint that records whenever KVM eagerly splits a huge page
and the error status of the split to indicate if it succeeded or failed
and why.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220119230739.2234394-18-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>