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

usr.sbin/bhyve/aarch64/vmexit.c

@@ -249,7 +249,8 @@ vmexit_smccc(struct vmctx *ctx, struct vcpu *vcpu, struct vm_run *vmrun)
 			how = VM_SUSPEND_POWEROFF;
 		else
 			how = VM_SUSPEND_RESET;
-		vm_suspend(ctx, how);
+		error = vm_suspend(ctx, how);
+		assert(error == 0 || errno == EALREADY);
 		break;
 	default:
 		break;

usr.sbin/bhyve/bhyverun.c

@@ -413,7 +413,8 @@ fbsdrun_addcpu(int vcpuid)
 
 	CPU_SET_ATOMIC(vcpuid, &cpumask);
 
-	vm_suspend_cpu(vi->vcpu);
+	error = vm_suspend_cpu(vi->vcpu);
+	assert(error == 0);
 
 	error = pthread_create(&thr, NULL, fbsdrun_start_thread, vi);
 	assert(error == 0);