pc: make pc_dimm_plug() more readble

split addr initialization from declaration so that
later when new local vars are added property getter
wouldn't drift off of error check.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
