LoongArch: KVM: Fix PMU pass-through issue if VM exits to host finally

In function kvm_pre_enter_guest(), it prepares to enter guest and check
whether there are pending signals or events. And it will not enter guest
if there are, PMU pass-through preparation for guest should be cancelled
and host should own PMU hardware.

Cc: stable@vger.kernel.org
Fixes: f4e40ea9 ("LoongArch: KVM: Add PMU support for guest")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>