KVM: x86: Open code read vs. write userspace MMIO exits in emulator_read_write()

Open code the differences in read vs. write userspace MMIO exits instead
of burying three lines of code behind indirect callbacks, as splitting the
logic makes it extremely hard to track that KVM's handling of reads vs.
write is _significantly_ different.  Add a comment to explain why the
semantics are different, and how on earth an MMIO write ends up triggering
an exit to userspace.

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/20260225012049.920665-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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