bhyvectl: Generalize bhyve IPC code

[bnovkov]

Summary

Move the nvlist-based bhyve IPC code into a separate function.
No functional change intended.

[rew]

if possible, it might be worth splitting the ipc changes outside of hot pluggable stack

think you can land the ipc changes without the hot plugging bits

usr.sbin/bhyvectl/bhyvectl.c

257–258

if you rename nvl to request and then see my suggestion for lines 278-285

290–305

could use nvlist_xfer() instead of nvlist_send() and nvlist_recv()

declare a nvlist_t *reply and then something like reply = nvlist_xfer(socket_fd, request, 0)

[bnovkov]

Address @rew 's comments.

usr.sbin/bhyvectl/bhyvectl.c

290–305

Thank you for the suggestion, nvlist_xfer makes things much simpler.