kvm tool: Implement a serial FIFO emulation

kvm tool: Implement a serial FIFO emulation

Adding FIFO functionality to the serial emulator to speed up the
console handling.

This also restricts sysrq functionality to the first serial port and
fixes the sysrq injection, which was broken because nothing ever reset
the BI status bit in the LSR.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Pekka Enberg <penberg@kernel.org>