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")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 4558c11f1b4dfd7fd505d70b79467eb7f1193f07)