Sasha reported, that a non RT guest reports "too much work for irq 4"
with the previous serial overhaul.

The reason is, that the new code allows unlimited tx transfers, which
triggers the sanity check in the 8250.c interrupt handler.

Limit the consecutive TX chars to 16 and let the guest kernel escape
from the 8250 interrupt handler. Set the TEMT/THRE bits in the
periodic serial console update.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Pekka Enberg <penberg@kernel.org>