bhyvectl: Add '--get-debug-cpus' command

vmm and libvmmapi already have handlers for that. When adding debug
cpus, they were only used for the debug stub. Over time, they were
reused by other parts like snapshots or idle APs.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D40804

(cherry picked from commit 65f8467e3351c38a9d57b538b85cf6c5fab5818e)