bhyve: export passthru softc

[corvink]

Summary

The gpu passthrough emulation will be handled in a seperate source file.
Therefore, export the passthru softc to give the gpu passthrough
emulation access to softc members.

[rew]

Seems a bit odd making this public? as opposed to keeping a reference of the softc in gpu_passthru and passing that off to pci?

something like:

gpu_passthru.c:

struct gpu_passthru {
   struct pci_passthru *passthru_sc;
}

struct gpu_passthru *gp_sc;
passthru_read(gp_sc->passthru_sc);

just a thought - its a pattern I've seen in other areas of the code.

[corvink]

I've checked my gpu passthrough code and it requires access to some internal fields of passthru_softc. We can add some helper to access them. So, this patch isn't required.