plugins.xunit: correct the read size from the back of the file

When attaching only partial log file we already calculate the "size =
max_size / 2", therefor it does not make sense to divide it twice (it
actually results in even shorter attached logs.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/plugins/xunit.py

@@ -76,7 +76,7 @@ class XUnitResult(Result):
                         text_output = logfile_obj.read(size)
                         text_output += ("\n\n--[ CUT DUE TO XML PER TEST "
                                         "LIMIT ]--\n\n")
-                        logfile_obj.seek(-size / 2, 2)
+                        logfile_obj.seek(-size, 2)
                         text_output += logfile_obj.read()
                 else:
                     text_output = logfile_obj.read()