s390/pci: embedding hotplug_slot in zdev

Embedding the hotplug_slot in zdev structure allows to
greatly simplify the hotplug handling by eliminating
the handling of the slot_list.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>