core: Fix test break in UpdateVmDisksCommandTest

Fixed the test break caused by commit
b0e796daa33a5c2e693bc785c064b81dc7e9e79f.

Change-Id: Ifa0f74d5ebe8aa33b70ce9cb3772d8ce5c894cbb
Signed-off-by: Allon Mureinik <amureini@redhat.com>