xend: pass-through: use devfn instead of slots as the unit for pass-through

xend: pass-through: use devfn instead of slots as the unit for pass-through

Instead of suppling a slot number to qemu-xen, supply a devfn.

This and subsequent other changes will allow xend to ask
for more than one function to be inserted into a single slot -
by specifying which function of the slot should be used.

This is a minimal patch for this change. A subsequent
patch that has a lot of noise to rename slot to devfn
is intended to follow.

Cc: Dexuan Cui <dexuan.cui@intel.com>
Cc: Masaki Kanno <kanno.masaki@jp.fujitsu.com>
Signed-off-by: Simon Horman <horms@verge.net.au>