riscv: Implement Guest/VM arch functions

This patch implements all kvm__arch_<xyz> Guest/VM arch functions.

These functions mostly deal with:
1. Guest/VM RAM initialization
2. Updating terminals on character read
3. Loading kernel and initrd images

Firmware loading is not implemented currently because initially we
will be booting kernel directly without any bootloader. In future,
we will certainly support firmware loading.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Link: https://lore.kernel.org/r/20211119124515.89439-4-anup.patel@wdc.com


Signed-off-by: Will Deacon <will@kernel.org>