OvmfPkg/RiscVVirt/README: explain that "-bios" is only useful with TCG

Riscv64 KVM services SBI calls internally, so OpenSBI is not used in such
setups. Restrict the "-bios" flag accordingly. Thanks: Andrei, Drew.

Cc: Andrei Warkentin <andrei.warkentin@intel.com>
Cc: Andrew Jones <ajones@ventanamicro.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: '-bios' is rejected with KVM accel (Drew)]
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>

OvmfPkg/RiscVVirt/README.md

@@ -86,3 +86,8 @@ then specify that binary for QEMU, with the following additional command line
 option:
 
     -bios $OPENSBI_DIR/build/platform/generic/firmware/fw_dynamic.bin
+
+Note that the above only makes a difference with software emulation (which you
+can force with `-M accel=tcg`). With hardware virtualization (`-M accel=kvm`),
+KVM services the SBI (Supervisor Binary Interface) calls internally, therefore
+any OpenSBI binary specified with `-bios` is rejected.