[various_mem_balloons]: define mem_attrs in the cfg

1.mem_attrs are not defined in the cfg file,
which is being fetched in various_mem_balloons.py
2.Hence added the value for mem_attrs in cfg

Signed-off-by: meshetty <meshetty@redhat.com>

Author: meshetty

libvirt/tests/cfg/memory/memory_balloon/various_mem_balloons.cfg

@@ -12,6 +12,7 @@
     memballoon_alias_name = "ua-c80aba6e-b6d8-448b-ab6e-8c7b5c29f353"
     memballoon_device_str = "balloon"
     sized_event_type = "yes"
+    mem_attrs = {'memory_unit':'KiB','memory':${mem_value},'current_mem':${current_mem},'current_mem_unit':'KiB'}
     variants:
         - virtio:
             memballoon_model = "virtio"