Fix list disks for the case where [vm-name] argument is provided

vm disk list fails to retrieve disk when a vm-name is provided. 

Repro steps:

$ azure vm disk list myvmname

Current:

info:    Executing command vm disk list
-error:   The server failed to authenticate the request. Verify that the certificate is valid and is associated with this subscription.
info:    Error information has been recorded to azure.err
error:   vm disk list command failed

Expected:

info:    Executing command vm disk list
+ Getting virtual machines
data:    Lun  Size(GB)  Blob-Name
data:    ---  --------  ----------------------------------
data:         76        myrandomimage.vhd
info:    vm disk list command OK