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

bhyve: Move device model-independent UART code into a separate file

Currently bhyve implements a ns16550-compatible UART in uart_emul.c.
This file also contains generic code to manage RX FIFOs and to handle
reading from and writing to a TTY. bhyve instantiates UARTs to
implement COM devices (via pci_lpc.c) and PCI UART devices.

The arm64 port will bring with it a PL011 device model which is used as
the default console (i.e., no COM ports). To simplify its integration,
add a UART "backend" layer which lets UART device models allocate an RX
FIFO and interact with TTYs without duplicating code. In particular,
code in uart_backend.* is to be shared among device models, and the
namespace for uart_emul.* is changed to uart_ns16550_*.

This is based on andrew@'s work in
https://github.com/zxombie/freebsd/tree/bhyvearm64 but I've made a
number of changes, particularly with respect to naming and source code
organization.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40993