xend: pass-through: Only tell qemu-xen to unplug function 0

xend: pass-through: Only tell qemu-xen to unplug function 0

When unplugging a function, all functions in the same vslot must be
unplugged, and function 0 must be one of the functions present when a
vslot is hot-plugged. Telling qemu-dm to unplug function 0 also tells
it to unplug all other functions in the same vslot.

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