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

[gusev.vitaliy_gmail.com]

Summary

vmm and libvmmapi already have handlers for that, but bhyvectl doesn't, by unknown reason.

Sponsored by: vStack

Test Plan

Run VM with 4 vcpus and get it into GRUB menu.

bhyvectl --get-debug-cpus --vm=ubuntu22
debug cpus:	 1, 2, 3

# bhyvectl --get-all --vm=ubuntu22|grep debug
debug cpus:	 1, 2, 3

bhyvectl --get-active-cpus --vm=ubuntu22
active cpus:	 0, 1, 2, 3

[jhb]

Originally the debug_cpus mask was only used for vCPUs stopped while the debug stub was active and in a stop, so it didn't seem worth adding to bhyvectl at the time. Over time it has been reused for other things (snapshots and now idle APs).