floss: wire hid_virtual_unplug from btclient

Some PTS tests requires us to send a HID virtual unplug, which we have
as libbluetooth API but there's no way to access them from btclient.

This CL wires that functionality.

Bug: 344778226
Test: m -j
Test: Run PTS HID/HOS/HCR/BV-03-C
Flag: EXEMPT, floss only changes
Change-Id: Ib85829f20c78a16068c7bddacbba31c8cfc19ad4