Improve errors from tar-in/tgz-in commands (RHBZ#591155 RHBZ#591250).

Improve errors from tar-in/tgz-in commands (RHBZ#591155 RHBZ#591250).

This commit improves the error messages from the tar-in, tgz-in (etc)
commands by capturing the stderr from the tar command in a file and
sending that back in the error message.

The method used for the error file is primitive, and there is a case
for a more generic error file mechanism, but this will do for now.

Sample error messages after this change:

$ virt-tar -u /tmp/test1.img /tmp/not.tar /
tar_in: tar subcommand failed on directory: /: tar: This does not look like a tar archive
tar: Skipping to next header
tar: Exiting with failure status due to previous errors at /home/rjones/d/libguestfs/tools/virt-tar line 247.

$ virt-tar -u /tmp/test1.img /tmp/test.tar /
tar_in: tar subcommand failed on directory: /: tar: access.log: Cannot open: Read-only file system
tar: Exiting with failure status due to previous errors at /home/rjones/d/libguestfs/tools/virt-tar line 247.