Some PCI register of passthru devices require a special handling. E.
g. MSI registers are emulated by bhyve. Additionally, some PCI
devices like Intel's integrated graphics device require that some
PCI registers are handled in a non standard way. Instead of using
many if's for each special case, use different handlers. Each
passthru device has an own set of handlers.
At the moment, this patch only uses handlers to emulate some PCI
header register. It can be expanded to use a special handler for
cmd/msi/... register accesses in the future.