vl: delay thread initialization after daemonization

Commit ac4119c (chardev: Use timer instead of bottom-half to postpone
open event, 2012-10-12) moved the alarm timer initialization to an earlier
point but failed to consider that it depends on qemu_init_main_loop.

Later, commit 1c53786 (vl: init main loop earlier, 2012-10-30) fixed
this, but left -daemonize in two different ways.  First, timers need to
be reinitialized after forking.  Second, the global mutex was being held
by the parent, and thus dropped after forking.

The first is now fixed using pthread_atfork.  For the second part,
make sure that the global mutex is not taken before daemonization,
and similarly delay qemu_thread_self.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
