Add Unplug v2 interface (REV_0900000A)

Add Unplug v2 interface (REV_0900000A)

Unplug v2 adds a query call to determine if a device type has had its unplug issued.
This is useful during upgrade cases when the Unplug keys have been set to 0, and can
be used to prevent XenVif from starting whilst emulated devices are present, but those
emulated devices have not been assigned a valid configuration yet (emulated devices
will receive valid configuration, but not at this point in the startup sequence during
upgrade)

Signed-off-by: Owen Smith <owen.smith@cloud.com>
Renamed UnplugWasRequested() to UnplugIsRequested() in the XENBUS_UNPLUG_INTERFACE_V2
by renaming the clashing XEN_API function from UnplugIsRequested() to UnplugGetRequest()

Signed-off-by: Paul Durrant <pdurrant@amazon.com>