Allow PCI VGA devices to be detached.

[grehan]

Summary

GPUs often have VGA PCI device IDs and are probed/attached
by the VGA driver. Allow them to be detached so they can
be presented as passthru devices to VM guests.

[jhb]

sys/dev/pci/vga_pci.c

290 ↗

(On Diff #42069)

Cosmetic nits. Existing style in this file is to use 'error' instead of 'rc' and to not do assignments inside of conditionals, so:

int error;

error = bus_generic_detach(dev);
if (error == 0)
    errror = device_delete_children(dev);
return (error);

[rgrimes]

How do you trigger a call to this DEVMETHOD?

[imp]

In D15269#321869, @rgrimes wrote:

How do you trigger a call to this DEVMETHOD?

1. devctl detatch vga0

[grehan]

Update style as per jhb.