passthrough: multi-function PCI in ACPI: rename slot to devfn

passthrough: multi-function PCI in ACPI: rename slot to devfn

This is a companion to the previous patch "qemu-xen: pass-through:
multi-function PCI in ACPI". It does not make any functional changes,
rather it just renames slot to devfn as appropriate.

Cc: Dexuan Cui <dexuan.cui@intel.com>
Cc: Masaki Kanno <kanno.masaki@jp.fujitsu.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
[7/8; cross-compatibility issues with xen-unstable.hg]