xend: pass-through: Parse command line for multi-function hot-plug and unplug

xend: pass-through: Parse command line for multi-function hot-plug and unplug

Hook things up to allow multi-function pass-through.

This includes making sure that request is valid.
In the case of pci-detach:

* All the functions requested must be attached to the same virtual
  slot * and;
* A request must include the functions attached to a virtual slot

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