Don't fail to read a file because it's non-seekable (e.g., a pipe).

Don't fail to read a file because it's non-seekable (e.g., a pipe).

* src/util.c (fread_file_lim): New function.
(__virFileReadAll): Use fread_file_lim, rather than requiring
that stat.st_size provide a usable file size.
* tests/read-non-seekable: New test, for the above.
* tests/Makefile.am (test_scripts): Add read-non-seekable.
* tests/test-lib.sh (mkfifo_or_skip_): New helper function.