qemu-char: Permit only a single "stdio" character device

When more than one is used, the terminal settings aren't restored
correctly on exit.  Fixable.  However, such usage makes no sense,
because the users race for input, so outlaw it instead.

If you want to connect multiple things to stdio, use the mux
chardev.

Signed-off-by: Li Liu <john.liuli@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
