virtio-serial-bus: move qdev properties into virtio-serial-bus.c

virtio-serial-bus: move qdev properties into virtio-serial-bus.c

As only one place in virtio-serial-bus.c uses
DEFINE_VIRTIO_SERIAL_PROPERTIES, there is no need to expose it. Inline
it into virtio-serial-bus.c to avoid wrongly use.

Signed-off-by: Shannon Zhao <[email protected]>
Signed-off-by: Shannon Zhao <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Acked-by: Cornelia Huck <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>