core: use local TX in AddVm flow

Taking VDS calls out of TX in Add VM Pool with VMs flow so long calls to
VDSM won't exceed the TX timeout.

To understand the commands that are influenced by this concider this
call tree:

 <pre>
 AddVmPoolWithVMsCommand
  |
   -> Lock tepmlate status
  |
   -> AddVmAndAttachToPoolCommand
       |
        -> AddVMCommand
            |
             -> save static,dynamic etc.   (new TX + compensation)
            |
             -> GetImageInfo - VDSM call   (out of TX)
            |
             -> set image status           (new TX + compensation)
            |
             -> CreateSnapshot - VDSM call (out of TX)
            |
             -> update old|save new disk image (new TX + compensation)

Bug-Url: https://bugzilla.redhat.com/948282
Change-Id: If93240cca8bee4988f28c3de574b44c9db31e8f9
Signed-off-by: Roy Golan <rgolan@redhat.com>