[QEMU-DM] Upgrade emulated UART to 16550A.

[QEMU-DM] Upgrade emulated UART to 16550A.

This patch adds 16550 emulation to qemu-dm. I still consider it a work
in progress, but from my testing, it consistently performs better than
the old code already, sometimes considerably so (on my laptop, dumping
out data over serial from a HVM DomU -> pty in Dom0 was up to 5.3
times faster than with the old 16450 code). I can consistenly reach
full 115200 baud speeds over physical serial lines when the virtual
device is backed by a phys port, which I never could with the old code.

There are still some things to be done. I want to add proper error
handling, so that overflow/framing & parity errors on the physical
port get detected and reported by the virtual port. Also, now that
FIFOs are in place, I believe performance could be improved
significantly by putting some code into the hypervisor so that reads &
writes from the FIFOs don't have to exit into qemu-dm at all. I'm also
reading up on the specs for newer uarts with deeper FIFOs.

Signed-off-by: Trolle Selander <trolle.selander@gmail.com>