error: Simplify error sink setup

qemu_error_sink can either point to a monitor or a file.  In practice,
it always points to the current monitor if we have one, else to
stderr.  Simply route errors to the current monitor or else to stderr,
and remove qemu_error_sink along with the functions to control it.

Actually, the old code switches the sink slightly later, in
handle_user_command() and handle_qmp_command(), than it gets switched
now, implicitly, by setting the current monitor in monitor_read() and
monitor_control_read().  Likewise, it switches back slightly earlier
(same places).  Doesn't make a difference, because there are no calls
of qemu_error() in between.

vl.c

@@ -4864,7 +4864,6 @@ int main(int argc, char **argv, char **envp)
 
     init_clocks();
 
-    qemu_errors_to_file(stderr);
     qemu_cache_utils_init(envp);
 
     QLIST_INIT (&vm_change_state_head);