RISC-V: KVM: Implement guest external interrupt line management

The RISC-V host will have one guest external interrupt line for each
VS-level IMSICs associated with a HART. The guest external interrupt
lines are per-HART resources and hypervisor can use HGEIE, HGEIP, and
HIE CSRs to manage these guest external interrupt lines.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>