audio VTS: Limit mix ports that route to attached devices

In tests that do not request and activate simulation of
connection of external devices, limit the mix port query to
return ports that can be routed to attached device ports only.

Bug: 205884982
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I7e77b269c07b763a5080f42ebf7ec7d0fcaee195