fix a potential segfault in LvmVg::doCreate

actual effects depend on l->chunkSize()

LvmVg.cc:1677:40: warning: adding 'unsigned long long' to a string does not append to the string
      [-Wstring-plus-int]
                cmd += " --chunksize " + l->chunkSize();
                       ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
LvmVg.cc:1677:40: note: use array indexing to silence this warning
                cmd += " --chunksize " + l->chunkSize();
                                       ^
                       &               [               ]