kvmtool: virtio: enable arm/arm64 support for bi-endianness

Implement the kvm_cpu__get_endianness call for both AArch32 and
AArch64, and advertise the bi-endianness support.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>