Revert "hw/virtio/virtio-pmem: Replace impossible check by assertion"

This reverts commit 5960f254db since the
previous commit made this situation possible again.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

hw/virtio/virtio-pmem.c

@@ -147,7 +147,10 @@ static void virtio_pmem_fill_device_info(const VirtIOPMEM *pmem,
 static MemoryRegion *virtio_pmem_get_memory_region(VirtIOPMEM *pmem,
                                                    Error **errp)
 {
-    assert(pmem->memdev);
+    if (!pmem->memdev) {
+        error_setg(errp, "'%s' property must be set", VIRTIO_PMEM_MEMDEV_PROP);
+        return NULL;
+    }
 
     return &pmem->memdev->mr;
 }