KVM: x86: Rename .read_write_emulate() to .read_write_guest()

KVM: x86: Rename .read_write_emulate() to .read_write_guest()

Rename the ops and helpers to read/write guest memory to clarify that they
do exactly that, i.e. aren't generic emulation flows and don't do anything
related to emulated MMIO.

Opportunistically add comments to explain the flow, e.g. it's not exactly
obvious that KVM deliberately treats "failed" accesses to guest memory as
emulated MMIO.

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/20260225012049.920665-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>