KVM: x86: Dedup kvm_sev_es_mmio_{read,write}()

KVM: x86: Dedup kvm_sev_es_mmio_{read,write}()

Dedup the SEV-ES emulated MMIO code by using the read vs. write emulator
ops to handle the few differences between reads and writes.

Opportunistically tweak the comment about fragments to call out that KVM
should verify that userspace can actually handle MMIO requests that cross
page boundaries.  Unlike emulated MMIO, the request is made in the GPA
space, not the GVA space, i.e. emulation across page boundaries can work
generically, at least in theory.

No functional change intended.

Tested-by: Tom Lendacky <thomas.lendacky@gmail.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260225012049.920665-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>