bhyve: define array to protect passthru regs

[corvink]

Summary

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 commit, we can start to split the default handler
into several handler for each register that requires emulation.

[markj]

usr.sbin/bhyve/pci_passthru.c

661

[markj]

usr.sbin/bhyve/pci_passthru.c

883

Alternately, can cfgread/write just call cfgread/write_default if no handler is installed? That might be a bit simpler and easier to follow.

[corvink]

• call cfgread/write_default if no handler is installed

[markj]

usr.sbin/bhyve/pci_passthru.c

883

But now this call isn't needed, right? And set_pcir_handler() can assert that a handler is not already set?

[corvink]

• assert at set_pcir_handler if handler is already set