bhyve: Generalize the IPC thread

[bnovkov]

Summary

Move the code for the snapshotting IPC thread into a separate file
and define macros for adding new IPC commands.
No functional change intended.

[rew]

only suggestions, not a request

usr.sbin/bhyve/ipc.c

61–75

what do you think about each ipc command constructing their own nvlist as a reply?

and then let handle_message() destroy the passed in nvl list?

there are two different class errors that can occur here, one is that the requested command is not supported and the second class is the handler encountered an error...might be worth being able to differentiate between those two different types of errors...only mentioning, your decision

99–111

looking at the suggested edit...then handle_message() would be responsible for destroying nvl and constructing the reply message.

[bnovkov]

Address @rew 's comments - change IPC command return type to nvlist_t.

[bnovkov]

usr.sbin/bhyve/ipc.c

61–75

Yeah, having the handler return a "reply" object makes more sense than reusing the one passed as an argument.
Thank you for the suggestion!

[bnovkov]

• constify command nvlist_t argument
• adapt vm_do_checkpoint to use the new return type