hw/riscv/virt.c: calculate socket count once in create_fdt_imsic()

hw/riscv/virt.c: calculate socket count once in create_fdt_imsic()

riscv_socket_count() returns either ms->numa_state->num_nodes or 1
depending on NUMA support. In any case the value can be retrieved only
once and used in the rest of the function.

This will also alleviate the rename we're going to do next by reducing
the instances of MachineState 'mc' inside hw/riscv/virt.c.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230124212234.412630-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>