LoongArch: KVM: Add vm migration support for LBT registers

Every vcpu has separate LBT registers. And there are four scr registers,
one flags and ftop register for LBT extension. When VM migrates, VMM
needs to get LBT registers for every vcpu.

Here macro KVM_REG_LOONGARCH_LBT is added for new vcpu lbt register type,
the following macro is added to get/put LBT registers.
  KVM_REG_LOONGARCH_LBT_SCR0
  KVM_REG_LOONGARCH_LBT_SCR1
  KVM_REG_LOONGARCH_LBT_SCR2
  KVM_REG_LOONGARCH_LBT_SCR3
  KVM_REG_LOONGARCH_LBT_EFLAGS
  KVM_REG_LOONGARCH_LBT_FTOP

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