qemu-char: cleanup after completed conversion to cd->create

qemu-char: cleanup after completed conversion to cd->create

All backends now return errors through Error*, so the "Failed to
create chardev" placeholder error can only be reached if the backend
is not available (and only from the chardev-add QMP command; instead,
the -chardev command line option fails earlier).

Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>