target/riscv: Enable the Hypervisor extension by default

Let's enable the Hypervisor extension by default. This doesn't affect
named CPUs (such as lowrisc-ibex or sifive-u54) but does enable the
Hypervisor extensions by default for the virt machine.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220105213937.1113508-7-alistair.francis@opensource.wdc.com>