vmm: Add credential to cdev object

Add a credential to the cdev object in sysctl_vmm_create(), then check
that we have the correct credentials in sysctl_vmm_destroy(). This
prevents a process in one jail from opening or destroying the /dev/vmm
file corresponding to a VM in a sibling jail.

Add regression tests.

Reviewed by: jhb, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31156