qemu-char: convert stdio backend to data-driven creation

qemu-char: convert stdio backend to data-driven creation

The backend now always returns errors via the Error* argument.
This avoids a double error message.  Before:

    qemu-system-x86_64: -chardev stdio,id=base: cannot use stdio with -daemonize
    qemu-system-x86_64: -chardev stdio,id=base: Failed to create chardev

After:

    qemu-system-x86_64: -chardev stdio,id=base: cannot use stdio with -daemonize

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