hw/intc/imsic: refine the IMSIC realize

hw/intc/imsic: refine the IMSIC realize

When the IMSIC is emulated in the kernel, the GPIO output lines to CPUs
and aia_ireg_rmw_fn setting can be remove. In this case the IMSIC
trigger CPU interrupts by KVM APIs, and the RMW of IREG is handled in
kernel.

This patch also move the code that claim the CPU interrupts to the
beginning of IMSIC realization. This can avoid the unnecessary resource
allocation before checking failed.

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250224025722.3999-2-yongxuan.wang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>