bhyve: Fix the WITH_BHYVE_SNAPSHOT build

Note, this breaks compatibility with snapshots generated by older builds
of bhyve(8).

Fixes: 7fa233534736 ("bhyve: Map the MSI-X table unconditionally for passthrough")
Reported by: Greg V <greg@unrelenting.technology>
Reviewed by: grehan, bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32523