[Crossport] Check return values from g_poll and select

The current implementation of os_host_main_loop_wait() on Windows,
returns 1 only when a g_poll() event occurs because the return value of
select() is overridden. This is wrong as we may skip a socket event, as
shown in this example:

1. select() returns 0
2. g_poll() returns 1  (socket event occurs)
3. os_host_main_loop_wait() returns 1
4. qemu_iohandler_poll() sees no socket event because select() has
   return before the event occurs
5. select() returns 1
6. g_poll() returns 0 (g_poll overrides select's return value)
7. os_host_main_loop_wait() returns 0
8. qemu_iohandler_poll() doesn't check for socket events because the
   return value of os_host_main_loop_wait() is zero.
9. goto 5

This patch use one variable for each of these return values, so we don't
miss a select() event anymore.

Also move the call to select() after g_poll(), this will improve latency
as we don't have to go through two os_host_main_loop_wait() calls to
detect a socket event.
[Crossport] main-loop: fix select_ret uninitialized variable warning
[Crossport] main-loop: switch to g_poll() on POSIX hosts

Use g_poll(3) instead of select(2).  Well, this is kind of a cheat.
It's true that we're now using g_poll(3) on POSIX hosts but the *_fill()
and *_poll() functions are still using rfds/wfds/xfds.

We've set the scene to start converting *_fill() and *_poll() functions
step-by-step until no more rfds/wfds/xfds users remain.  Then we'll drop
the temporary gpollfds_from_select() and gpollfds_to_select() functions
and be left with native g_poll(2).

On Windows things are a little crazy: convert from rfds/wfds/xfds to
GPollFDs, back to rfds/wfds/xfds, call select(2), rfds/wfds/xfds back to
GPollFDs, and finally back to rfds/wfds/xfds again.  This is only
temporary and keeps the Windows build working through the following
patches.  We'll drop this excessive conversion later and be left with a
single GPollFDs -> select(2) -> GPollFDs sequence that allows Windows to
use select(2) while the rest of QEMU only knows about GPollFD.
[Crossport] main-loop: switch POSIX glib integration to GPollFD

Convert glib file descriptor polling from rfds/wfds/xfds to GPollFD.

The Windows code still needs poll_fds[] and n_poll_fds but they can now
become local variables.
[Crossport]  g_strdup(NULL) returns NULL; simplify
[Crossport] iohandler: switch to GPollFD

Convert iohandler_select_fill() and iohandler_select_poll() to use
GPollFD instead of rfds/wfds/xfds.
[Crossport] main-loop: drop rfds/wfds/xfds for good

Now that all *_fill() and *_poll() functions use GPollFD we no longer
need rfds/wfds/xfds or pollfds_from_select()/pollfds_to_select().

>From now on everything uses GPollFD.
Disable slirp in qemu-kvm
Disable feature xfsctl