Nova has two pages in documentation listing things supported on several
architectures/hypervisors. This patch adds initial state of AArch64
into support matrix.
Document minimal qemu/libvirt for aarch64. Version 3.6.0 was first one
which worked for us with Nova without a need for extra patches.
Change-Id: I2ee7be9e88e20ed0f77be07fed4fdd800533b3c5