KVM: x86: Harden SEV-ES MMIO against on-stack use-after-free

Add a sanity check to ensure KVM doesn't use an on-stack variable when
handling an MMIO request for an SEV-ES guest.  The source/destination
for SEV-ES MMIO should _always_ be the #VMGEXIT scratch area.

Opportunistically update the comment in the completion side of things
to clarify that frag->data doesn't need to be copied anywhere, and the
VMEGEXIT is trap-like (the current comment doesn't clarify *how* RIP is
advanced).

Tested-by: Tom Lendacky <thomas.lendacky@gmail.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260225012049.920665-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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