qemu-char: fix deadlock with "-monitor pty"

qemu_chr_be_generic_open cannot be called with the write lock taken,
because it calls client code that may call qemu_chr_fe_write.  This
actually happens for the monitor:

    0x00007ffff27dbf79 in __GI_raise (sig=sig@entry=6)
    0x00007ffff27df388 in __GI_abort ()
    0x00005555555ef489 in error_exit (err=<optimized out>, msg=msg@entry=0x5555559796d0 <__func__.5959> "qemu_mutex_lock")
    0x00005555558f9080 in qemu_mutex_lock (mutex=mutex@entry=0x555556248a30)
    0x0000555555713936 in qemu_chr_fe_write (s=0x555556248a30, buf=buf@entry=0x5555563d8870 "QEMU 2.0.90 monitor - type 'help' for more information\r\n", len=56)
    0x00005555556217fd in monitor_flush_locked (mon=mon@entry=0x555556251fd0)
    0x0000555555621a12 in monitor_flush_locked (mon=0x555556251fd0)
    monitor_puts (mon=mon@entry=0x555556251fd0, str=0x55555634bfa7 "", str@entry=0x55555634bf70 "QEMU 2.0.90 monitor - type 'help' for more information\n")
    0x0000555555624359 in monitor_vprintf (mon=0x555556251fd0, fmt=<optimized out>, ap=<optimized out>)
    0x0000555555624414 in monitor_printf (mon=<optimized out>, fmt=fmt@entry=0x5555559105a0 "QEMU %s monitor - type 'help' for more information\n")
    0x0000555555629806 in monitor_event (opaque=0x555556251fd0, event=<optimized out>)
    0x000055555571343c in qemu_chr_be_generic_open (s=0x555556248a30)

To avoid this, defer the call to an idle callback, which will be
called as soon as the main loop is re-entered.  In order to simplify
the cleanup and do it in one place only, change pty_chr_close to
call pty_chr_state.

To reproduce, run with "-monitor pty", then try to read from the
slave /dev/pts/FOO that it creates.

Fixes: 9005b2a7589540a3733b3abdcfbccfe7746cd1a1
Reported-by: Li Liang <liangx.z.li@intel.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
