passthrough: update bus2bridge mapping as PCI devices get added/removed

passthrough: update bus2bridge mapping as PCI devices get added/removed

This deals with two limitations at once: On device removal, the
mapping did not get updated so far at all, and hotplugged devices as
well as such not discoverable by Xen's initial bus scan (including the
case where a non-zero PCI segment wasn't accessible during Xen boot,
but became accessible after Dom0 validated access information against
ACPI data) wouldn't cause updates to the mapping either.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: "Kay, Allen M" <allen.m.kay@intel.com>