New goldfish_pipe virtual device.

This adds a new virtual hardware device named "goldfish_pipe"
used to implement a very fast communication channel between the
guest system and the emulator.

IMPORTANT: This depends on a special kernel driver, see:

    https://review.source.android.com/#change,22496

Usage from the guest is simply the following:

   fd = open("/dev/qemu_pipe", O_RDWR);
   const char* pipename = "pipe:<name>";
   ret = write(fd, pipename, strlen(pipename)+1);
   if (ret < 0) {
        /* could not connect to service named <name> */
   }
   /* now you can read()/write()/close() as a normal
    * file descriptor to exchange data with the service.
    */

In addition, this implements the following pipe services in the
emulator:

   tcp:<port>
   tcp:<hostname>:<port>
   unix:<path>
   opengles

The 'tcp:' and 'unix:' services simply redirect to a TCP or Unix
socket on the host with minimal

The 'opengles' service simply connects to tcp:locahost:22468 for now.
We may change this to be more configurable in the future, but that's
the port number used by the current experimental OpenGL ES hardware
emulation host libraries / programs.

Benchmarking with a simple ping-pong program shows that the
guest <-> emulator can achieve a roundtrip bandwidth of 192 MB/s
(on a 2.7 Ghz Xeon PC).

Using the tcp: service to talk to a ping-pong server listening
on localhost reaches 102 MB/s on the same machine, using a Unix
socket reaches 140 MB/s.

By contrast, using standard sockets in the guest reaches only
3.8 MB/s on the same machine (and requires special privileges
from the application anyway).

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