LoongArch: KVM: Rework kvm_send_pv_ipi()

LoongArch: KVM: Rework kvm_send_pv_ipi()

The function in fact traverses a "bitmap" stored in GPR regs A1 and A2,
but does it in a non-obvious way by creating a single-word bitmap twice.

This patch switches the function to create a single 2-word bitmap, and
also employs for_each_set_bit() macro, as it helps to drop most of the
housekeeping code.

While there, convert the function to return void to not confuse readers
with unchecked result.

Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>