unmodified_drivers: enable unplug per default

unmodified_drivers: enable unplug per default

Since xen-3.3 an official unplug protocol for emulated hardware is
available in the toolstack. The pvops kernel does the unplug per
default, so it is safe to do it also in the drivers for forward ported
xenlinux.
Currently its required to load xen-platform-pci with the module
parameter dev_unplug=all, which is cumbersome.
Also recognize the dev_unplug=never parameter, which provides the
default before this patch.

Signed-off-by: Olaf Hering <olaf@aepfle.de>