vmm: Avoid infinite loop in vcpu_lock_all error case.

Reported by: Coverity (CIDs 1501060,1501071)
Reviewed by: corvink, markj, emaste
Differential Revision: https://reviews.freebsd.org/D37648

(cherry picked from commit d212d6ebb4ea3b3e9c3964c1a6d3f41817e437e1)