RISC-V: Enable QEMU virt machine support in defconfigs

We have kconfig option for QEMU virt machine so let's enable it
in RV32 and RV64 defconfigs. Also, we remove various VIRTIO configs
from RV32 and RV64 defconfigs because these are now selected by
QEMU virt machine kconfig option.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>