Fix vdi can't load.

[corentindrouet]

Check if there is a machine before taking it username and password, for prevent crash (username of undefine).

To test:

• Launch a vdi.
• Create an image.
• Close the vdi.
• Launch a vdi on the new image

Expected result:

• The vdi is launched correctly.

[coveralls]

Changes Unknown when pulling 6916096caa1363d02d7ea27031934a52d113c388 on corentindrouet:fix_crash_vdi into \ on Nanocloud:master**.