ARM: vGIC: cache virtual LPI priority in struct pending_irq

ARM: vGIC: cache virtual LPI priority in struct pending_irq

We enhance struct pending_irq to cache the priority information
for LPIs. Reading the information from there is faster than accessing
the property table from guest memory. Also it use some padding area in
the struct, so does not require more memory.
This introduces the function to retrieve the LPI priority as a vgic_ops.
Also this moves the vgic_get_virq_priority() call in
vgic_vcpu_inject_irq() to happen after the NULL check of the pending_irq
pointer, so we can rely on the pointer in the new function.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>