bhyve: Move lock of uart frontend to uart backend

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