Don't warn about missing UUIDs for non-existent containers.

It's not possible for the container or its members to have UUIDs
until they have been created.

blivet/devices/container.py

@@ -84,7 +84,7 @@ def _verifyMemberUuid(self, member, expect_equality=True, require_existence=True
         member_fmt_uuid = getattr(member.format, self._formatUUIDAttr)
 
         # If either UUID can not be obtained, nothing to check.
-        if not member_fmt_uuid or not self.uuid:
+        if self.exists and (not member_fmt_uuid or not self.uuid):
             log.warning("At least one UUID missing.")
             return None