core: deadlock and unneeded db/vdsm calls  when creating diskless snapshots.

This patch removes uneeded vdsm and db calls in case of an diskless
snapshots and also solves a occuring deadlock.

1. No need to call endSuccesfully method when creating diskless
snapshots at the moment, the call to endSuccesfully can perform uneeded
vdsm call if the VM is running.

2. When creating diskless snapshot, a new transaction is opened within
endSuccesfully, calling endSuccesfully in case that we already run in
transaction will cause that we won't see newly added data from the new
transaction and FK caused deadlock in the db, as a new
snapshot that references a vm is inserted(acquires FK db lock on the vm
row) and later on we try to update the same row from the newly created
transaction thus creating a deadlock.

Change-Id: I2ca72a2b7b881827493ddec5c53bd2c485a74084
Signed-off-by: Liron Aravot <laravot@redhat.com>