bhyve: add ROM emulation

Some PCI devices especially GPUs require a ROM to work properly.
The ROM is executed by boot firmware to initialize the device.
To add a ROM to a device use the new ROM option for passthru device
(e.g. -s passthru,0/2/0,rom=<path>/<to>/<rom>).

It's necessary that the ROM is executed by the boot firmware.
It won't be executed by any OS.
Additionally, the boot firmware should be configured to execute the
ROM file.
For that reason, it's only possible to use a ROM when using
OVMF with enabled bus enumeration.

Differential Revision: https://reviews.freebsd.org/D33129
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 month