LoongArch: KVM: Move host CSR_EENTRY save and restore in context switch

LoongArch: KVM: Move host CSR_EENTRY save and restore in context switch

CSR register LOONGARCH_CSR_EENTRY is shared between host CPU and guest
vCPU, KVM need save and restore LOONGARCH_CSR_EENTRY register. Here move
LOONGARCH_CSR_EENTRY saving in to context switch function rather than VM
entry.

At most time VM enter/exit is much more frequent than vCPU thread context
switch.

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