xm: xen-api, pass-through: create: Use vslot for hotplug_slot

xm: xen-api, pass-through: create: Use vslot for hotplug_slot

Using func for hotplug_slot is not correct, although func is often
zero, previously zero meant please pick a vslot and asking xend to
pick a vslot was the only method available.

This resolves the following error when using Xen API:
$ xm create hvm.conf
...
Internal error: Timed out waiting for device model action.

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