vmm: Use make_dev_s() to create vmm devices

This avoids creating windows where a device file is accessible but the
device-specific field is not set.

Now that vmmdev_mtx is a sleepable lock, avoid dropping it while
creating devices files. This makes it easier to handle races and
simplifies some code; for example, the VSC_LINKED flag is no longer
needed.

Suggested by: jhb
Reviewed by: imp, jhb
Differential Revision: https://reviews.freebsd.org/D46488