hw/riscv: Fix type conflict of GLib function pointers

hw/riscv: Fix type conflict of GLib function pointers

qtest_set_command_cb passed to g_once should match GThreadFunc,
which it does not.  But using g_once is actually unnecessary,
because the function is called by riscv_harts_realize() under
the Big QEMU Lock.

Reported-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250410161722.595634-1-pbonzini@redhat.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: qemu-stable@nongnu.org
(cherry picked from commit 56cde18d048e1e1f889e31f7553e1f39f03eeec5)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>