The --quiet argument is not available on all operating systems.  Use -s
instead to match the rest of the Makefile uses.  This fixes a non-fatal
error seen on FreeBSD.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Sean Bruno <sbruno@freebsd.org>
Message-Id: <20160614180734.8782-1-sbruno@freebsd.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>