arm64: defconfig: add virtio support for running as a kvm guest

When running as a kvm guest on a para-virtualised platform, it is useful
to have virtio implementations of console, 9pfs and network.

This adds these options to the arm64 defconfig, so we can easily run a
defconfig kernel build as both host and as a kvm guest.

Signed-off-by: Will Deacon <will.deacon@arm.com>