resctrl: Use exclusive lock for /sys/fs/resctrl

resctrl: Use exclusive lock for /sys/fs/resctrl

That's the way it should've been all the time.  It was originally the case, but
then the rework to virFileFlock() made the function ambiguous when it was
created in commit 5a0a5f7fb5f5, and due to that it was misused in commit
657ddeff2313 and since then the lock being taken was shared rather than
exclusive.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>