Fix bugs in loading plugin set

Fix bugs in loading plugin set

Fixes in the LoadPluginSet() function:
- the naming scheme for backend libraries is "libTYPE.so", where TYPE
  is the backend type (e.g. for tpk it should load libtpk.so)
- elements of the global plugin_set_list array should be allocated
  every time a new backend library is loaded as this behaviour is
  expected by the backend api: pkg_plugin_on_load() function expects
  its input parameter to be an already allocated memory area, otherwise
  it returns -1 and the LoadPluginSet() reports a failure:

    extern "C" int pkg_plugin_on_load(pkg_plugin_set* set) {
      if (set == nullptr)
        return -1;
      set->plugin_on_unload = PluginOnUnload;
      ...
    }

Change-Id: I603030871a28f050f1d87ce572c102c3d0b5dc90