core: fix removing snapshot's memory on remove vm

On remove VM, the memory volumes in snapshots are removed after the
snapshots of the VM are removed, so the memory volumes should be
removed if no other snapshot use them (instead of checking whether
there's one snapshot that use them).

Change-Id: I500d1abb1b040887a595a58f1797eb19b82189db
Bug-Url: https://bugzilla.redhat.com/960931
Signed-off-by: Arik Hadas <ahadas@redhat.com>