bhyve: Correct unmapping of the MSI-X table BAR

The starting address passed to mprotect was wrong, so in the case where
the last page containing the table is not the last page of the BAR, the
wrong region would be unmapped.

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