core: AddDiskCommand - added disk can never be a disk snapshot

When adding a disk the added disk can never be a disk snapshot,
Currently when adding DiskImages the disk's Image object is "active"
member has a default value of false, which leads to wrongly treat that
disk as a disk snapshot through validations rather than a regular added
disk.

Change-Id: I13c3c819a25bd33e5443d82728423c0de7087326
Signed-off-by: Liron Aravot <laravot@redhat.com>