dimm: Correct type of MemoryHotplugState->base

The 'base' field of MemoryHotplugState is ram_addr_t, which indicates that
it exists in the abstract address space of RAM regions.

However, the actual usage of this field indicates that it is a concrete
physical address (it's passed as an offset to memory_region_add_subgregion
for example).

So, correct its type to 'hwaddr'.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com>