xen/arm: vgic: Introduce a new field to store the rank index and use it

xen/arm: vgic: Introduce a new field to store the rank index and use it

Having in hand the index for the rank is very handy to avoid computing
it every time.

For now, use it when enabling/disabling the vIRQs rather than a formula
which is not obvious to understand.

Also drop the comments which were wrong because a shift by DABT_WORD
will not give the IRQ number but the index of the register.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>