pc,pc-dimm: Extract hotplug related fields in PCMachineState to a structure

pc,pc-dimm: Extract hotplug related fields in PCMachineState to a structure

Move hotplug_memory_base and hotplug_memory fields of PCMachineState
into a separate structure so that the same can be made use of from
other architectures supporing memory hotplug.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>