core: RemoveDiskParameters - don't use Entity ID

Changed RemoveDiskParameters to stop using Entity ID (which should only
be used by Async Tasks related classes) and instead added a diskId
member.

All current usages were fixed to use the new member.

Change-Id: I70dabec5d542c343cbaf71b1a4fdb58b4ec970ff
Signed-off-by: Allon Mureinik <amureini@redhat.com>