target/loongarch: Move some function definition to kvm directory

Move function definition specified with kvm to the corresponding
directory. Also remove header file "cpu.h" including outside of
macro QEMU_KVM_LOONGARCH_H.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>