webadmin: Pass created_by_user_id field on VM Edit

Explicitly set created_by_user_id field to the original (unchanged)
value for VM upon edit to avoid backend error due to change to
non-editable field.

Change-Id: I129c7e2c48b1fda5cdd1719e5f6a1b95ec46843d
Signed-off-by: Martin Betak <mbetak@redhat.com>