hostapd(vts): Add a property for SAP interface

Changes to the VTS test to support OEM's who use a separate
dedicated SAP interface.

Note: This is a VTS test change only. The default implementation does
not still support a dedicated SAP interface (OEM's can obviously
customize the default implementation for their devices).

Bug: 136645414
Test: None (verified by partner OEM)
Change-Id: I42b87bf90cfa90c7c218309164ab2b77f3685f9e