pc-dimm: add busy slot check and slot auto-allocation

- if slot property is not specified on -device/device_add command,
treat default value as request for assigning PCDIMMDevice to
the first free slot.

- if slot is provided with -device/device_add command, attempt to
use it or fail command if it's already occupied.

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>
