engine: Change vm_guest_interface_type.interface_name to text

The interface name as reported by the guest agent is not bounded
for Window OS guests, therefore its type should be modified to text.

Change-Id: I9ac9bb8125f9c00bf930fa2b7408efbd7df66521
Bug-Url: https://bugzilla.redhat.com/893361
Signed-off-by: Moti Asayag <masayag@redhat.com>