vmm: Remove stale comment for vm_rendezvous.

Support for rendezvous outside of a vcpu context (vcpuid of -1) was
removed in commit 949f0f47a4e7, and the vm, vcpuid argument pair was
replaced by a single struct vcpu pointer in commit d8be3d523dd5.

Reported by: andrew

(cherry picked from commit 1f6db5d6b5de5e0cafcdb141a988120b0faea049)