bhyve: define array to protect passthru regs

GPU passthrough requires a special handling of some PCI config register.
Therefore, we need a flexible approach for implementing it. Adding an
array of handler meets this condition.

Start by using the default handler for all accesses to the PCI config
space. In upcoming commits, we can start to split the default handler
into several handler for each register that requires emulation.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39291