Rewrite LXC I/O forwarding to use main event loop

Rewrite LXC I/O forwarding to use main event loop

The current I/O code for LXC uses a hand crafted event loop
to forward I/O between the container & host app, based on
epoll to handle EOF on PTYs. This event loop is not easily
extensible to add more consoles, or monitor other types of
file descriptors.

Remove the custom event loop and replace it with a normal
libvirt event loop. When detecting EOF on a PTY, disable
the event watch on that FD, and fork off a background thread
that does a edge-triggered epoll() on the FD. When the FD
finally shows new incoming data, the thread re-enables the
watch on the FD and exits.

When getting EOF from a read() on the PTY, the existing code
would do waitpid(WNOHANG) to see if the container had exited.
Unfortunately there is a race condition, because even though
the process has closed its stdio handles, it might still
exist.

To deal with this the new event loop uses a SIG_CHILD handler
to perform the waitpid only when the container is known to
have actually exited.

* src/lxc/lxc_controller.c: Rewrite the event loop to use
  the standard APIs.