xen/arm: vgic-v2: Don't ignore a write in ITARGETSR if one field is 0

xen/arm: vgic-v2: Don't ignore a write in ITARGETSR if one field is 0

The current implementation ignores the whole write if one of the field is
0. Although, based on the spec (4.3.12 IHI 0048B.b), 0 is a valid value
when:
    - The interrupt is not wired in the distributor. From the Xen
    point of view, it means that the corresponding bit is not set in
    d->arch.vgic.allocated_irqs.
    - The user wants to disable the IRQ forwarding in the distributor.
    I.e the IRQ stays pending in the distributor and never received by
    the guest.

Implementing the later will require more work in Xen because we always
assume the interrupt is forwarded to a valid vCPU. So for now, ignore
any field where the value is 0.

The emulation of the write access of ITARGETSR has been reworked and
moved to a new function because it would have been difficult to
implement properly the behavior with the current code.

The new implementation is breaking the register in 4 distinct bytes. For
each byte, it will check the validity of the target list, find the new
target, migrate the interrupt and store the value if necessary.

In the new implementation there is nearly no distinction of the access
size to avoid having too many different path which is harder to test.

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