bhyve: add bootindex option for several devices

The bootindex option creates an entry in the "bootorder" fwcfg file.
This file can be picked up by the guest firmware to determine the
bootorder. Nevertheless, it's not guaranteed that the guest firmware
uses the bootorder. At the moment, our OVMF ignores the bootorder. This
will change in the future.

If guest firmware supports the "bootorder" fwcfg file and no device uses
the bootindex option, the boot order is determined by the firmware
itself. If one or more devices specify a bootindex, the first bootable
device with the lowest bootindex will be booted. It's not garanteed that
devices without a bootindex will be recognized as bootable from the
firmware in that case.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39285