The following is a short write-up of how I accomplished converting my mythtv master-backend to a virtual machine. I did this for the purpose of hardware flexibility, which allowed me to run my backend on my fileserver while I upgraded it's hardware and OS.
This should apply to almost any linux version, but the target machine I performed this on was Fedora 6.
It was performed on a running machine.
Step 1: Prepare the Virtual Disk
I used qemu to create an image of a particular type. Raw is easiest to deal with for native block device handling and it can be converted to another format later.
In my case I created this device on a storage partition that wasn't being converted to the virtual machine. You can easily use an NFS or SMB mount point if need be.