stand: Add devformat to return formatted string for a device

Use dv_fmtdev to return a formatted string for a device. If this is a
null pointer, return the device name and unit followed by a colon (eg
disk3:).

Sponsored by: Netflix
Reviewed by: tsoome (prior version)
Differential Revision: https://reviews.freebsd.org/D35916

(cherry picked from commit dc472f67021320309ced970d3d26d30a04da0ef2)