bhyve/kdblayout: add some missing keys to german layout

The '/' and '§' keys are missing in the german keyboard layout.

Reviewed by: markj
Approved by: manu (mentor)
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37386

(cherry picked from commit eff8d0347713c52bed70179610df1be7c8badbe6)