vmm: Add credential to /dev/vmm.io cdev

[cyril_freebsdfoundation.org]

Summary

The commit a85404906bc8 added a credential for the /dev/vmm cdev but not for /dev/vmm.io. This revision adds the missing credential.

[markj]

sys/amd64/vmm/vmm_dev.c

1231

I think it'd be a bit nicer to change devmem_create_cdev() to pass the vmmdev_softc in directly. This function is static, so its usage is confined to this file and there's no real reason to avoid modifying its signature if it makes the code simpler.

[cyril_freebsdfoundation.org]

Simplify code.