target/loongarch: Add post init function for kvm mode

target/loongarch: Add post init function for kvm mode

Some features such as LBT and PMU are implemented in kvm mode,
With paravirt features in future, post init function is added
for kvm mode, so that property for these features will be created
in kvm post init function.

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