KVM: Add function comments for __kvm_read/write_guest_page()

The (gfn, data, offset, len) order of parameters is a little strange
since "offset" applies to "gfn" rather than to "data". Add function
comments to make things perfectly clear.

Signed-off-by: Anish Moorthy <amoorthy@google.com>
Link: https://lore.kernel.org/r/20240215235405.368539-3-amoorthy@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>