core: cleanup AttachDettachVmDiskParameters c'tor

AttachDettachVmDiskParameters has two c'tors. Implementation was
separate although one could depend on the other, which is what happens
now.

Change-Id: Ifd3d5dd8adbfda6cf67f61b9f1fa2eb337fb6d73
Signed-off-by: Vered Volansky <vvolansk@redhat.com>