Update virttest/nfs.py

Co-authored-by: Nick Zuo <903627391@qq.com>

Author: Anushree-Mathur

virttest/nfs.py

@@ -196,7 +196,7 @@ def __init__(self, params):
             params.get("setup_remote_nfs") == "yes"
             or params.get("setup_local_nfs") == "yes"
         ):
-            if "Ubuntu" in distro_details or "rhel" in distro_details or "fedora" in distro_details:
+            if distro_details.upper() in ("UBUNTU", "RHEL", "FEDORA", ):
                 self.nfs_service = service.Service("nfs-server", session=self.session)
             else:
                 self.nfs_service = service.Service("nfs", session=self.session)