xen/arm: vgic-v3: Clean the emulation of IROUTER

xen/arm: vgic-v3: Clean the emulation of IROUTER

The read emulation of the register IROUTER contains lots of uncessary
code as irouter is already valid and doesn't need any processing before
setting the value in a register.

Also take the opportunity to factorize the code to find a vCPU from the
affinity in a single place. It will be easier to change the way to do it
later.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Cc: Chen Baozi <cbz@baozis.org>
Acked-by: Chen Baozi <baozich@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>