pc-dimm: do not allow setting an in-use memdev

using the same memdev backend more than once will cause
assertion at MemoryRegion mapping time because it's already
mapped. Prevent it by checking that the associated MemoryRegion
is not mapped.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MST: tweak commit log
