#8 QtEmu does not check if the disk image exists

[bughunter2]

When trying to run a virtual machine that doesn't have a virtual disk, it fails silently because the "disk image" parameter is still passed to QEMU but of course qemu will fail.

If no "disk image" is set QtEmu should not pass the corresponding parameter to QEMU. Also it should first check whether the disk image exists and display an error message if it doesn't.