bhyve: make passthru sel public available

The GVT-d emulation requires access to this selector to read from the
device.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40035

(cherry picked from commit 60793cee27993048864f60e3c19039386e4a4ab0)