core: remove duplicated custom properties handling

The logic of extracting the predefined and user properties from VM
custom properties was duplicated in some places. This patch keep only
the one which is located in VmPropertiesUtils and change the other
places to use it.

Change-Id: I4c37a001985da09cbb7e3f78db1e419901d532cf
Signed-off-by: Arik Hadas <ahadas@redhat.com>