core: Prepare custom device properties support in VmPropertiesUtils

1) Move org.ovirt.engine.core.utils.vmproperties.VmPropertiesUtils to
   org.ovirt.engine.core.utils.customprop package
2) Extract common functionality to abstract parent CustomPropertiesUtils
   to support custom device properties in following patches

Change-Id: Ib18a622a1d02f444c6bdc1e22c2f92f93c52757e
Signed-off-by: Martin Perina <mperina@redhat.com>