core: RemoveVm - fill quota release parameters when needed

*If the vm disks aren't going to be removed, there's no need to create
quota release parameters.

*The vm snapshots should be filled only once and not on every iteration.

*The operation should be done for the disks that would be removed only.

Change-Id: I666091dbc64b03b9cf19b76f26a4afb8100b8373
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1026083
Signed-off-by: Liron Aravot <laravot@redhat.com>