curses: don't initialize curses when qemu is daemonized

Current qemu initializes curses even if -daemonize option is
passed. This cause problem because shell prompt appears without
calling endwin().

This patch adds new function, is_daemonized(), to OS dependent
code. With this function, curses_display_init() can check that qemu is
daemonized or not. If daemonized, curses_display_init() isn't called
and the problem is avoided.

Of course, -daemonize && -curses doesn't make sense. Users shouldn't
pass the arguments at the same time. But the problem is very painful
because Ctrl-C cannot be delivered to the terminal.

Cc: Andrzej Zaborowski  <balrog@zabor.org>
Cc: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Hitoshi Mitake <h.mitake@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
