libvmmapi: Fix auto-loading of vmm.ko

[markj]

Summary

• We should autoload vmm.ko when creating a VM with vm_openf(), to preserve behaviour prior to commit 99127fd10362.
• kldload(2) returns a non-zero value upon success, so the existing code was wrong.

Reported by: olivier
Fixes: 99127fd10362 ("libvmmapi: Use the vmmctl device file to create and destroy VMs")

[markj]

I don't really like the auto-loading, but that's how it used to work, and changing it in 15.0 would be a POLA violation.