bhyve: Assert success in a couple of libvmmapi calls

• In vmexit_smccc(), copy an assertion from amd64.
• In fbsdrun_addcpu(), make sure that our vm_suspend_cpu() call is succesful.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D46197