Upgrade emulated UART to 16550A (Stefano Stabellini)

Upgrade emulated UART to 16550A (Stefano Stabellini)

This patch upgrades the emulated UART to 16550A, the code comes from
xen-unstable. The main improvement was introduced with the following patch and
subsequent email thread:

http://lists.xensource.com/archives/html/xen-devel/2007-12/msg00129.html

The changes compared to previous version are:

- change clock_gettime to qemu_get_clock

- no token bucket anymore;

- fixed a small bug handling IRQs; this was the problem that prevented
kgdb to work over the serial (thanks to Jason Wessel for the help
spotting and reproducing this bug).

- many many style fixes;

- savevm version number increased;

- not including termios.h and sys/ioctl.h anymore, declaring static
constants in qemu-char.h instead;

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4993 c046a42c-6fe2-441c-8c8c-71466251a162