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 <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>