So far, we require the hypervisor to update the VLPI properties
once the the VLPI mapping has been established. While this
makes it easy for the ITS driver, it creates a window where
an incoming interrupt can be delivered with an unknown set
of properties. Not very nice.

Instead, let's add a "properties" field to the mapping structure,
and use that to configure the VLPI before it actually gets mapped.
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>