core: add disk - storage domain not specified error message

AddDiskCommand -> canDo:
added a verification that a storage domain ID has been
specified by the client (and return an appropriate
error message accordingly).

Change-Id: Ibd74f8ecd23b3c01108c31a0bb46f99002fd7071
Bug-Url: https://bugzilla.redhat.com/1120768
Signed-off-by: Daniel Erez <derez@redhat.com>