Avoid risk of format string abuse (also avoids gcc warnings).
        * src/util.c (ReportError): Use a literal "%s" format string.
        * src/remote_internal.c (server_error): Likewise.
        * src/qemu_conf.c (qemudReportError): Likewise.
        * acinclude.m4: Add -Wformat -Wformat-security to default
          list of warning flags, to warn about errors such as the
          above.