hw/riscv/virt.c: use s->memmap in create_fdt_virtio()

hw/riscv/virt.c: use s->memmap in create_fdt_virtio()

create_fdt_virtio() can use s->memmap instead of having an extra
argument for it.

While we're at it rewrite it a little bit to avoid the clunky line in
'name' and code repetition:

- declare 'virtio_base' out of the loop since it never changes;
- declare a 'size' variable. Use it to calculate the address of the
  virtio device in an 'addr' variable;
- use 'addr' in the 'name' g_strdup_printf();
- use 'addr' and 'size' when creating the 'reg' property.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250429125811.224803-8-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>