android_pipe.c: clean-up the vaddr->paddr code

I commonised the code and used contemporary qemu functions. However for
upstream the kernel driver need to move to passing hwaddr's to
QEMU (pref using virt-io mechanisms).
android_pipe.c: use the same accessors as everything else
android_pipe.c: remove multiple if/else legs

Makes the code look cleaner...
android_pipe: document that this host ptr is opaque to us

You could probably translate this but you shouldn't.