vmm: Use make_dev_s() to create vmm devices

[markj]

Summary

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

[jhb]

Hmm, so nothing was checking for si_drv1 being NULL in the cdevsw methods? Oof.

[imp]

Yikes indeed! Nice catch. It likely was usually fine, but yikes!