libxc: check for null size file mapping

libxc: check for null size file mapping

Changed the error message when trying to map a null size file.
When doing `xl create` command, we get an Invalid Kernel error
when the file size is greater than zero. For zero length files, we are
falling in the mmap error, and we get an `Invalid parameter` error,
which is not explicit. With this change, we get a `zero length file`
error.

Signed-off-by: Paul Semel <semelpaul@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>