Do not write LVM devices file during image installation

Follow up for rhinstaller#5325 which disables the LVM devices file usage on
blivet side but we also need to skip writing the file when
writing the storage configuration post install.

Resolves: rhbz#2247872

pyanaconda/modules/storage/installation.py

@@ -343,6 +343,9 @@ def _write_lvm_devices_file(storage, sysroot):
         :param Blivet storage: instance of Blivet or a subclass
         :param str sysroot: path to the target OS installation
         """
+        if conf.target.is_image:
+            return
+
         if not HAVE_LVMDEVICES:
             return