disable qemu PCI devices in HVM domains

disable qemu PCI devices in HVM domains

Magic ioport (0x10) protocol for negotating with guest PV drivers
during startup, and allowing PV drivers to disable hardware emulations
thus preventing guest from seeing the same device through two paths.

Protocol and implementation from the Citrix Xenserver product line.
Documentation (protocol spec) will follow in a moment.

Contributed-By: Steven Smith <steven.smith@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>