DO NOT MERGE ANYWHERE Qemu: make the qemu_pipe_open compatible with old apis

Commit c7b098ce has changed
the qemu_pipe_open interface to require the "pipe:" prefix in
the service name.

However in APIs 24 and before, the "pipe:" prefix is not required
This causes quite some confusion and bugs since it is very common
to forget the difference when working across differnet APIs.

This CL is meant to make qemu_pipe_open work in both cases by
doing the following:
1. try the service name as is;
2. if it fails, add 'pipe:' prefix and try the service name again.

Change-Id: If9782396c03780fad1aadeb8374eb308517dc963