Emulator: enhance logcat -Q to handle qemu pipe device

Upon opening, qemu pipe (a.k.a. goldfish pipe) requires a purpose
string so that emulator can route the content to the right channel
on the host.

This CL will ask emulator to send the content to pipe based 'logcat'
service on the host.

Change-Id:  Icc71f81d5b95b64ea315fe10da82ff704416e449