bhyve: Move lock of uart frontend to uart backend

Currently, lock of uart in bhyve is placed in frontend. There are some
problems about it:

1. If every frontend should has a lock, why not move it inside backend as they all have same uart_softc.
2. If backend needs to modify the information of uart after initialize, it will be impossible as backend cannot use lock. For example, if we want implement a telnet support for uart in backend, It should wait for connection when initialize. After some remote process connect it, it needs to modify rfd and wfd in backend.

So I decide to move it to backend.

Reviewed by: corvink, jhb, markj
Differential Revision: https://reviews.freebsd.org/D44947