ixv(4): Allow PF to control the VF link state

This patch adds checks of a VF link state provided by PF via mailbox
API. Such change enables the PF to disable a VF administratively.

Since command needed by the PF to control the VF is introduced in
mailbox api version 1.2, this patch also bumps supported mailbox api
version to 1.2.

Co-authored-by: Krzysztof Galazka <krzysztof.galazka@intel.com>

Reviewed By: kbowling@
Tested by: lukasz.szczepaniak@intel.com
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D32004