loongarch/kvm: Remove SW timer switch when vcpu is halt polling

LoongArch inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8I8NK



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This patches removes SW timer switch during vcpu block stage. VM uses HW
timer rather than SW PV timer on LoongArch system, it can check pending
HW timer interrupt status directly, rather than switch to SW timer and
check injected SW timer interrupt.

When SW timer is not used in vcpu halt-polling mode, the relative
SW timer handling before entering guest can be removed also. Timer
emulation is simpler than before, SW timer emuation is only used in vcpu
thread context switch.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>