core: Moved disk alias genenration in AddDiskCommand to execution

Moved the disk alias generation in AddDiskCommand to the execution phase
as in the job properties calculation the VM disk map is not yet populated.
Also the disk alias was omitted from the execution message as we cannot
assure that in 100% of the times a disk alias will be specified (the VM
name is not there for the same reason)

Bug-Url: https://bugzilla.redhat.com/1168252
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Change-Id: Id30fa5d3a99dd12b8c45633d592d017c3a6ffd1b