arm64/vmm: Handle debug exits

This is enough to boot FreeBSD again.

PR: 290044
Reported by: Siva Mahadevan <me@svmhdvn.name>
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D52931

(cherry picked from commit 534aec00976c18c4c4b485b2fade9d5f33de89c8)