target-arm: Guest cpu endianness determination for virtio KVM ARM/ARM64

This patch implements a fucntion pointer "virtio_is_big_endian"
from "CPUClass" structure for arm/arm64.
Function arm_cpu_is_big_endian() is added to determine and
return the guest cpu endianness to virtio.
This is required for running cross endian guests with virtio on ARM/ARM64.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Message-id: 1423130382-18640-3-git-send-email-pranavkumar@linaro.org
[PMM: check CPSR_E in env->cpsr_uncached, not env->pstate.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
