Fix windows build and clean up use of <windows.h>

Fix windows build and clean up use of <windows.h>

We want to globally define WIN_LEAN_AND_MEAN and WINVER to particular values so
let's do it in OS_CFLAGS.

Then, we can pepper in windows.h includes where using #includes that require it.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6783 c046a42c-6fe2-441c-8c8c-71466251a162