hw/char/sh_serial: Add timeout handling to unbreak serial input

hw/char/sh_serial: Add timeout handling to unbreak serial input

As of commit 18e8cf159177100e ("serial: sh-sci: increase RX FIFO trigger
defaults for (H)SCIF") in Linux v4.11-rc1, the serial console on the
QEMU SH4 target is broken: it delays serial input until enough data has
been received.

Since aforementioned commit, the Linux SCIF driver programs the Receive
FIFO Data Count Trigger bits in the FIFO Control Register, to postpone
generating a receive interrupt until:
  1. At least the receive trigger count of bytes of data are available
     in the receive FIFO, OR
  2. No further data has been received for at least 15 etu after the
     last received data.

While QEMU implements the former, it does not implement the latter.
Hence the receive interrupt is not generated until the former condition
is met.

Fix this by adding basic timeout handling.  As the QEMU SCIF emulation
ignores any serial speed programming, the timeout value used conforms to
a default speed of 9600 bps, which is fine for any interactive console.

Reported-by: Rob Landley <rob@landley.net>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Ulrich Hecht <uli@fpond.eu>
Tested-by: Rob Landley <rob@landley.net>
Tested-by: Rich Felker <dalias@libc.org>
Message-Id: <20180905131125.12635-1-geert+renesas@glider.be>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>