passthrough: Read the interrupt pin from hardware

passthrough: Read the interrupt pin from hardware

Preparation for multi-fucntion devices in guests.

Up until now guests have only seen devices with one function,
so it is both safe and sensible to always use INTA. But if
if multiple functions appear, it makes sense to use
the interrupt pin specified by the hardware.

Cc: Dexuan Cui <dexuan.cui@intel.com>
Cc: Masaki Kanno <kanno.masaki@jp.fujitsu.com>
Cc: Yuji Shimada <shimada-yxb@necst.nec.co.jp>
Signed-off-by: Simon Horman <horms@verge.net.au>
[1/8; cross-compatibility issues with xen-unstable.hg]