KVM: x86: Trace unsatisfied MMIO reads on a per-page basis

Invoke the "unsatisfied MMIO reads" when KVM first detects that a
particular access "chunk" requires an exit to userspace instead of tracing
the entire access at the time KVM initiates the exit to userspace.  I.e.
precisely trace the first and/or second fragments of a page split instead
of tracing the entire access, as the GPA could be wrong on a page split
case.

Leave the completion tracepoint alone, at least for now, as fixing the
completion path would incur significantly complexity to track exactly which
fragment(s) of the overall access actually triggered MMIO, but add a
comment that the tracing for completed reads in is technically wrong.

Tested-by: Tom Lendacky <thomas.lendacky@gmail.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260225012049.920665-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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