qemu-doc: Fix executable name in examples

The executable name qemu was replaced some time ago by qemu-system-i386.
Fix all examples accordingly.

Some examples will only work with qemu-system-i386 or qemu-system-x86_64
for obvious reasons ("dos.img").

To keep things simple, I did not vary the executable name.
Place holders like qemu-system-TARGET were also only used once
in the enhanced description for QEMU launches using Wine.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
