hw/riscv/virt.c: fix the interrupts-extended property format of PLIC

hw/riscv/virt.c: fix the interrupts-extended property format of PLIC

The interrupts-extended property of PLIC only has 2 * hart number
fields when KVM enabled, copy 4 * hart number fields to fdt will
expose some uninitialized value.

In this patch, I also refactor the code about the setting of
interrupts-extended property of PLIC for improved readability.

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231218090543.22353-1-yongxuan.wang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>