pci: add a new pci_child_added newbus method.

This is needed so when running under Xen the calls to pci_child_added
can be intercepted and a custom Xen method can be used to register
those devices with Xen. This should not include any functional
change, since the Xen implementation will be added in a following
patch and the native implementation is a noop.

Sponsored by: Citrix Systems R&D
Reviewed by: jhb

dev/pci/pci.c:
dev/pci/pci_if.m:
dev/pci/pci_private.h:
dev/pci/pcivar.h:
 - Add the pci_child_added newbus method.
netback: fixes for netback

This patch contains the following fixes for netback:
 - Only unbind the evtchn if it has been bound.
 - Set xnb->bridge to NULL after free to prevent double-freeing it.
 - Set the MAC address for the host-facing interface to a dummy value.

Sponsored by: Citrix Systems R&D

dev/xen/netback/netback.c:
 - Prevent trying to unbind if the evtchn has not been bounded.
 - Prevent double-freeing xnb->bridge.
 - Set the MAC address of the host-facing interface to a dummy value,
   so it can work when the interface is added to a bridge.
vt(4): Give the window to vd_bitblt_text_t callback

... instead of both the buffer and the font. Again, this simplifies the
API.

MFC after:	1 week