Check fread() results to avoid gcc 4.6 warnings

When compiling with gcc 4.6, some code in fw_cfg.c complains that fop_ret
is assigned but not used (which is true).  However, it looks like the
meaningless assignments to fop_ret were done to suppress other gcc warnings
due to the fact that fread() is labelled as warn_unused_result in glibc.

This patch avoids both errors, by actually checking the fread() result code
and dropping out with an error message if it fails.

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
