bhyvectl: don't permit using --suspend and --checkpoint at same time
When using the --suspend and --checkpoint parameter, bhyvectl will
produce two checkpoint images and the exits. This is slightly ambiguous.
So, permit only one of theses parameters at the same time.
Reviewed by: corvink, markj
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D38887