RISC-V: KVM: Forward SBI DBCN extension to user-space

RISC-V: KVM: Forward SBI DBCN extension to user-space

The frozen SBI v2.0 specification defines the SBI debug console
(DBCN) extension which replaces the legacy SBI v0.1 console
functions namely sbi_console_getchar() and sbi_console_putchar().

The SBI DBCN extension needs to be emulated in the KVM user-space
(i.e. QEMU-KVM or KVMTOOL) so we forward SBI DBCN calls from KVM
guest to the KVM user-space which can then redirect the console
input/output to wherever it wants (e.g. telnet, file, stdio, etc).

The SBI debug console is simply a early console available to KVM
guest for early prints and it does not intend to replace the proper
console devices such as 8250, VirtIO console, etc.

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