Skip fstab mountpoints that are not valid directory paths

This commit aims to fix an issue where multiple swap partitions that have
the same 'none' or 'swap' as mountpoint. In order to achieve this, the linux
OSMount tools will skip any mountpoint that is not a valid directory.

coriolis/osmorphing/osmount/base.py

@@ -261,6 +261,13 @@ def _check_mount_fstab_partitions(
 
             device = match.group(2)
             mountpoint = match.group(3)
+
+            if not mountpoint.startswith('/'):
+                LOG.warning(
+                    f"Skipping mountpoint that is not a valid directory: "
+                    f"{mountpoint}")
+                continue
+
             if mountpoint in mounts:
                 raise exception.CoriolisException(
                     "Mountpoint '%s' appears to be mounted twice in "