Introduce @secure attribute to os loader element

Introduce @secure attribute to os loader element

This element will control secure boot implemented by some
firmwares. If the firmware used in <loader/> does support the
feature we must tell it to the underlying hypervisor. However, we
can't know whether loader does support it or not just by looking
at the file. Therefore we have to have an attribute to the
element where users can tell us whether the firmware is secure
boot enabled or not.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>