xen/arm: gic: don't use dom0 variable in gicv_setup

xen/arm: gic: don't use dom0 variable in gicv_setup

The commit 99bf30a "xen: arm: define guest virtual platform in API headers"
modifies gicv_setup to either use harcoded addresses for guest or hardware
addresses for dom0. In the latter case, the variable dom0 is not yet
initialized. dom0 receives assignment by the return of domain_create which
calls gicv_setup.
Use the ID to know whether the domain is dom0 or not.

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