core: Removing Memory Volumes Before VM and Disks

removeMemoryVolumes queries the VM and its disks and thus should come
before we remove them.
Note that removeMemoryVolumes can't be inside the transaction because
it's not a pure DB operation, but also contains a VDSM (SPM) call.

Change-Id: I24851385ab1c27a1da7fe8be04cfd9d8fb744b3f
Signed-off-by: Idan Shaby <ishaby@redhat.com>