xend: specify the slot for pass-through devices

xend: specify the slot for pass-through devices

Currently a slot may be specified for a hot-plug device,
but not for a pass-through device that is inserted at boot time.
This patch adds support for the latter.

The syntax is:
     BUS:DEV.FUNC[@VSLOT]
e.g: 0000:00:1d:0@7

This may be important as recent changes that allow any free PCI
slot to be used for pass-through (and hotplug) may case pass-through
devices to be assigned in different locations to before. Amongst
other things, specifying the slot will allow users to move them
back, if there is a need.

Signed-off-by: Simon Horman <horms@verge.net.au>