main: switch qemu_set_fd_handler to g_io_add_watch

This patch changes qemu_set_fd_handler to be implemented in terms of
g_io_add_watch().  The semantics are a bit different so some glue is required.

qemu_set_fd_handler2 is much harder to convert because of its use of polling.

The glib main loop has the major of advantage of having a proven thread safe
architecture.  By using the glib main loop instead of our own, it will allow us
to eventually introduce multiple I/O threads.

I'm pretty sure that this will work on Win32, but I would appreciate some help
testing.  I think the semantics of g_io_channel_unix_new() are really just tied
to the notion of a "unix fd" and not necessarily unix itself.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
