Fix the issue when var is empty but not None

Currently if var is empty but not None it will cause findmnt
cmd run into error, so update the code logic to fix this issue.
Also add some log output for the exception.

Signed-off-by: Leidong Wang <leidwang@redhat.com>

Author: leidwang

virttest/utils_disk.py

@@ -17,6 +17,7 @@
 import tempfile
 from functools import cmp_to_key
 
+import aexpect
 from avocado.core import exceptions
 from avocado.utils import process, wait
 from avocado.utils.service import SpecificServiceManager
@@ -64,7 +65,9 @@ def copytree(src, dst, overwrite=True, ignore=""):
             shutil.copy(src_file, dst_dir)
 
 
-def is_mount(src, dst=None, fstype=None, options=None, verbose=False, session=None):
+def is_mount(
+    src=None, dst=None, fstype=None, options=None, verbose=False, session=None
+):
     """
     Check is src or dst mounted.
 
@@ -77,23 +80,22 @@ def is_mount(src, dst=None, fstype=None, options=None, verbose=False, session=No
     :return: True if mounted, else return False
     """
     mount_options = [("-S", src), ("-M", dst), ("-t", fstype), ("-O", options)]
-    mount_opts = " ".join(
-        f"{opt} {val}" for opt, val in mount_options if val is not None
-    )
+    mount_opts = " ".join(f"{opt} {val}" for opt, val in mount_options if val)
     if mount_opts == "":
         raise exceptions.TestError("Mount options is empty, it is meaningless")
     mount_check_cmd = f"findmnt -J {mount_opts}"
 
     mount_result = None
     try:
         if session:
-            mount_result = session.cmd_output_safe(mount_check_cmd)
+            mount_result = session.cmd(mount_check_cmd)
         else:
             mount_result = process.run(mount_check_cmd, shell=True).stdout_text
         if verbose:
             LOG.info("Output of findmnt: %s", mount_result)
-    except process.CmdError:
-        pass
+    except (process.CmdError, aexpect.exceptions.ShellCmdError) as e:
+        LOG.error("Exception info: %s", e)
+        return False
 
     if mount_result:
         if verbose: