KVM: LoongArch: selftests: Add PMU overflow interrupt test

KVM: LoongArch: selftests: Add PMU overflow interrupt test

Extend the PMU test suite to cover overflow interrupts. The test enables
the PMI (Performance Monitor Interrupt), sets counter 0 to one less than
the overflow value, and verifies that an interrupt is raised when the
counter overflows. A guest interrupt handler checks the interrupt cause
and disables further PMU interrupts upon success.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>