passthrough: Add AUTO_PHP_DEVFN_MULTI

passthrough: Add AUTO_PHP_DEVFN_MULTI

Up until now there has been a signle magic value that xend can pass to
qemu-xen to signify that qemu-xen should choose a devfn for a pass-through
function.

When supporting multi-function devices in guests, it is useful for xend to
be able to sepcify if a function is to appear in a guest as a
single-function deveice, or as part of a multi-function device.

By adding AUTO_PHP_DEVFN_MULTI to suplement the existing AUTO_PHP_DEVFN,
this patch achieves that goal.

This patch does not break compatibility with xend as the value chosen for
AUTO_PHP_DEVFN_MULTI could never validly be sent by xend up until now.
However, there is a companion change to xend in order to make use of this
feature.

Cc: Dexuan Cui <dexuan.cui@intel.com>
Cc: Masaki Kanno <kanno.masaki@jp.fujitsu.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
[5/8; cross-compatibility issues with xen-unstable.hg]