vmm: Fix error handling in vmm_handler()

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.

Reviewed by: corvink, andrew
Fixes: a97f683fe3c4 ("vmm: Add a device file interface for creating and destroying VMs")
Differential Revision: https://reviews.freebsd.org/D48269