Sysinfo: make journalctl quiet on cursor acquisition

On environments where there are no journals available, journalctl may
output error messages such as "No journal files were found", unless
`--quiet` is given as an option.  The other call to journalctl already
does that, but we missed in the first call (that does the cursor
acquisition).
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/sysinfo.py

@@ -231,7 +231,7 @@ class JournalctlWatcher(Collectible):
 
     def _get_cursor(self):
         try:
-            cmd = 'journalctl --lines 1 --output json'
+            cmd = 'journalctl --quiet --lines 1 --output json'
             result = subprocess.check_output(cmd.split())
             last_record = json.loads(result)
             return last_record['__CURSOR']