hw/arm/virt: Bump the 255GB initial RAM limit

Now we have the extended memory map (high IO regions beyond the
scalable RAM) and dynamic IPA range support at KVM/ARM level
we can bump the legacy 255GB initial RAM limit. The actual maximum
RAM size now depends on the physical CPU and host kernel, in
accelerated mode. In TCG mode, it depends on the VCPU
AA64MMFR0.PARANGE.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20190304101339.25970-11-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>