error_report() prepends the location already.  Example:

$ qemu-img convert -6
qemu-img: qemu-img: option -6 is deprecated, please use '-o compat6' instead!
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>