Add a new PCI bus interface method to alloc the ivars (dinfo) for a device.

Add a new PCI bus interface method to alloc the ivars (dinfo) for a device.

The ACPI and OFW PCI bus drivers as well as CardBus override this to
allocate the larger ivars to hold additional info beyond the stock PCI ivars.

This removes the need to pass the size to functions like pci_add_iov_child()
and pci_read_device() simplifying IOV and bus rescanning implementations.

As a result of this and earlier changes, the ACPI PCI bus driver no longer
needs its own device_attach and pci_create_iov_child methods but can use
the methods in the stock PCI bus driver instead.

Differential Revision:	https://reviews.freebsd.org/D5891