restapi:  Cannot remove custom properties from a vm

When passing <custom_properties/> to update vm, the custom properties on
the vm aren't removed.
Just when passing - <custom_properties> <custom_property/>
</custom_properties> the custom properties are removed.

The mapper shouldn't ignore the custom properties if
vm.isSetCustomProperties()==true (Even if
vm.getCustomProperties().isSetCustomProperty()==false).

Change-Id: I9ba6f59eb08bc33b0f93848f9cd1bcba360605e1
Bug-Url: https://bugzilla.redhat.com/967874
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>