virNetDevSaveNetConfig: Pass mode to virFileWriteStr()

virNetDevSaveNetConfig: Pass mode to virFileWriteStr()

For some types of SRIOV interfaces we create a temporary file
where the state of the interface is saved before we start
modifying it. The file is used then to restore the original
configuration when the interface is no longer associated with any
guest. For writing the file virFileWriteStr() is used. However,
it's given wrong argument: the last argument is supposed to be
mode to create the file with but virNetDevSaveNetConfig() passes
open(2) flags (O_CREAT|O_TRUNC|O_WRONLY). We need the file to be
writable and readable by root only (0600). Therefore, pass that
mode instead of gibberish.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>