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$ qemu-img create fedora.qcow 5G
Of course you are not obliged to take 5GB.
Note: Even if you take 10GB this does NOT mean that the image does really HAVE the size of 10GB. It just means that your new system is limited up to 10GB - if the new system takes only 1,2 GB also the image will only be at 1,2GB.
Now let's install the OS. Put in the install CD and type into your konsole (all in one line without break):
$ qemu -cdrom /dev/cdrom -hda fedora.qcow -boot d -net nic -net user -m 196 -rtc base=localtime
"-user -net" is important to have internet access within your new system. "-m 196" is the Set virtual RAM size (megabytes), default is 128 MB, I chose 196.
The install may take some time. After the install, qemu will try to boot the new OS itself. Maybe this may fail (was the case for me) - but don't worry. If that happens: just close the qemu window and type the following command into your konsole to launch your new OS:
$qemu fedora.qcow -boot c -net nic -net user -m 196 -rtc base=localtime
Testing ISO Images
Type, in the proper directory
$ qemu -m 512M -cdrom <isoname>.iso
Debugging
To get kernel output dumped to a file outside the virtual system, add e.g. "-serial file:/tmp/qemu-output.log" to the qemu command line. When booting the virtual system, add "console=ttyS0" to the kernel boot parameters.
This output is particularly helpful if you are having trouble booting the system, in which case you may also wish to remove "rhgb" and "quiet" from the kernel boot parameters.