vmm: Fix wrong assert in ivhd_dev_add_entry

The correct condition is to check the number of ivhd entries fit into
the array.

Reported by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31514

(cherry picked from commit 179bc5729dd72e0f4252c0dce72454c76782f935)