vmm: Fix error handling in vmm_handler()

[markj]

Summary

In commit a97f683fe3c4 I didn't add code to remove the vmmctl device
when vmm.ko is unloaded, so it would persist and prevent vmm.ko from
being re-loaded.

Extend vmmdev_cleanup() to destroy the vmmctl cdev. Also call
vmmdev_cleanup() if vmm_init() fails.

Fixes: a97f683fe3c4 ("vmm: Add a device file interface for creating and destroying VMs")

[andrew]

sys/dev/vmm/vmm_dev.c

1003

Should this return error?

[markj]

Fix error return

[jrtc27]

sys/riscv/vmm/vmm.c

272

This comment persists but the arm64 equivalent was removed

[markj]

Remove the todo comment from riscv as well.