virttest.utils_test.libvirt: update disk source of the VM

To update disk source path of the VM according to specified direcotry, such as a NFS sharing directory.

Signed-off-by: Alex Jia <ajia@redhat.com>
Yes, I may set a variable for different disk source type, but at present, this function only work on 'file' or 'dev' type of the source, in other words, it's avaiable for file or block type disk and doesn't work on 'network' or 'volume' type disk.