vm/qemu: Run riscv64 kernel using 4-level page table

Riscv is far from having a hw with a 5-level support, so let's focus on the
4-level.

Author: Alexandre Ghiti

vm/qemu/qemu.go

@@ -191,7 +191,7 @@ var archConfigs = map[string]*archConfig{
 	},
 	"linux/riscv64": {
 		Qemu:                   "qemu-system-riscv64",
-		QemuArgs:               "-machine virt",
+		QemuArgs:               "-machine virt -cpu rv64,sv48=on",
 		NetDev:                 "virtio-net-pci",
 		RngDev:                 "virtio-rng-pci",
 		UseNewQemuImageOptions: true,