bhyve: use a fixed 32 bit BAR base address

OVMF always uses 0xC0000000 as base address for 32 bit PCI MMIO space.
For that reason, we should use that address too.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D31051
Sponsored by: Beckhoff Automation GmbH & Co. KG