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

(cherry picked from commit 062f2818c1ad35bdc3d520c7572e3cefda657770)