bhyve: Map the right BAR in init_msix_table()

The PBA and MSI-X table can reside in different BARs.

Reported by: Andy Fiddaman <andy@omniosce.org>
Reviewed by: jhb
Fixes: 7fa233534736 ("bhyve: Map the MSI-X table unconditionally for passthrough")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33739