hw/arm/virt: Introduce finalize_gic_version()

hw/arm/virt: Introduce finalize_gic_version()

Let's move the code which freezes which gic-version to
be applied in a dedicated function. We also now set by
default the VIRT_GIC_VERSION_NO_SET. This eventually
turns into the legacy v2 choice in the finalize() function.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20200311131618.7187-4-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>