stand: Use devformat rather than disk_devfmt

Fix layering violation and use devformat to get the string
representation of the device to see if we're mounted yet or not. Remove
added include to pickup disk.h.

Sponsored by: Netflix
Reviewed by: tsoome (prior version)
Differential Revision: https://reviews.freebsd.org/D35919

[ minor changes because UFS changes interferred and were already merged ]

(cherry picked from commit ec9f3e776f39286ccec9915f38cca9729e6f9241)