bhyve: emulate reads of MSI-X capabilities for passthru devices

Reads of the MSI-X capabilites aren't emulated by passthru devices
yet. The guest will read the host MSI-X capabilites which could
cause issues.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D32686
Sponsored by: Beckhoff Automation GmbH & Co. KG