xen: register xen pci notifier

Register a pci notifier to add (or remove) pci devices to Xen via
hypercalls. Xen needs to know the pci devices present in the system to
handle pci passthrough and even MSI remapping in the initial domain.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Qing He <qing.he@intel.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>