KVM: x86: Open code handling of completed MMIO reads in emulator_read_write()

KVM: x86: Open code handling of completed MMIO reads in emulator_read_write()

Open code the handling of completed MMIO reads instead of using an ops
hook, as burying the logic behind a (likely RETPOLINE'd) indirect call,
and with an unintuitive name, makes relatively straightforward code hard
to comprehend.

Opportunistically add comments to explain the dependencies between the
emulator's mem_read cache and the MMIO read completion logic, as it's very
easy to overlook the cache's role in getting the read data into the
correct destination.

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/20260225012049.920665-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>