Emulator: make qemu_pipe_open usage consistent with definition

Emulator: make qemu_pipe_open usage consistent with definition

The qemu_pipe_open(const char* purpose) inside the goldfish/include/qemu_pipe.h
adds 'pipe:' prefix to the purpose string, and the caller should not add that
'pipe:' prefix to avoid double prefixes of 'pipe:'

Note: This definition of qemu_pipe_open is the legacy definition; and it is different
from the new definition in system/core/include/qemu_pipe.h, where the caller should
add the 'pipe:' prefix. AFAIK, this new definition is used in adbd and rild.

goldfish project sticks with legacy definition for ease of cherry picking of fixes
accross different api levels.

Change-Id: I7b61bc1c58d64dcec5129582bd0981a4d534d084