libvirt.test: Fix a bug for update-device test

Disk maybe doesn't contain source element, use
'disk.source' will get LibvirtXMLNotFoundError.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #197 from liyangfnst/update_device_fix

libvirt.test: Fix a bug for update-device test