kvm: x86: fix comment about {mmu,nested_mmu}.gva_to_gpa

The comment had the meaning of mmu.gva_to_gpa and nested_mmu.gva_to_gpa
swapped. Fix that, and also add some details describing how each translation
works.

Signed-off-by: David Matlack <dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>