[libvirt] [PATCH]: don't harcode buffer for getgrnam_r

Hi, determines the maximum needed buffersize for getgrnam_r using sysconf instead of hardcoding it to 1024 and increases the buffer on ERANGE. The latter is needed since sysconf is allowed to return -1. Furthermore some glibc versions seem to return a too small buffer on amd64 (http://bugs.debian.org/520744). O.k. to apply? Cheers, -- Guido