char: Allow devices to use a single multiplexed chardev.

This fixes regression caused by commit
2d6c1ef40f3678ab47a4d14fb5dadaa486bfcda6
("char: Prevent multiple devices opening same chardev"):

-nodefaults -nographic -chardev stdio,id=stdio,mux=on,signal=off \
 -mon stdio -device virtio-serial-pci \
 -device virtconsole,chardev=stdio -device isa-serial,chardev=stdio

fails with:

qemu-system-x86_64: -device isa-serial,chardev=stdio: Property 'isa-serial.chardev' can't take value 'stdio', it's in use

Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
