MAINTAINERS: list qemu-security@nongnu.org as security contact

MAINTAINERS: list qemu-security@nongnu.org as security contact

The qemu-security@nongnu.org list is considered the authoritative
contact for reporting QEMU security issues. Remove the Red Hat
security team address in favour of QEMU's list, to ensure that
upstream gets first contact. There is a representative of the
Red Hat security team as a member of qemu-security@nongnu.org
whom requests CVE assignments on behalf of QEMU when needed.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Mauro Matteo Cascella <mcascell@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>