vmm: Validate credentials when opening a vmmdev
Rather than performing privilege checks after a specific VM's device
file is opened, do it once at the time the device file is opened. This
means that one can continue to access a VM via its device fd after
attaching to a jail which does not have vmm enabled, but this seems like
a reasonable semantic to have anyway.
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D46486