bhyvectl: Operations --suspend and --checkpoint are alternative

[gusev.vitaliy_gmail.com]

Summary

Currently it is allowed to specify command line as

'bhyvectl --suspend=A --checkpoint=B'

It will trigger a bhyve process produces two checkpoint images
and then exit. This is slightly ambiguous.

So allow only --suspend or --checkpoint at the same time.

Sponsored by: vStack

Test Plan

Compile bhyvectl.

Run

bhyvectl --suspend=A --checkpoint=B

verify it is not allowed.

2. Run bhyvectl --suspend=A
   
   verify it works.

3. Run bhyvectl --checkpoint=B
   
   verify it works.