disk-image-create: fix test when no argument is given.

Avoid a warning when multiple arguments are passed to disk-image-create
like:

bin/disk-image-create: 146: [: aa: unexpected operator

Change-Id: I619d7bf2e4b56885d629019b80b249bc80b8f43e
Merge "disk-image-create: fix test when no argument is given."