stand: Add dv_devfmt to return a string represenation of the device

Add a new pointer, dv_devfmt, to allow devices to format themselves. We
will use this to simplify many of the fmtdev functions in the tree as
they are all almost the same, or all are isomorphic to each other.

Sponsored by: Netflix
Reviewed by: tsoome (prior version)
Differential Revision: https://reviews.freebsd.org/D35915