target/loongarch/kvm: Add software breakpoint support

target/loongarch/kvm: Add software breakpoint support

With KVM virtualization, debug exception is injected to guest kernel
rather than host for normal break intruction. Here hypercall
instruction with special code is used for sw breakpoint usage,
and detailed instruction comes from kvm kernel with user API
KVM_REG_LOONGARCH_DEBUG_INST.

Now only software breakpoint is supported, and it is allowed to
insert/remove software breakpoint. We can debug guest kernel with gdb
method after kernel is loaded, hardware breakpoint will be added in later.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Tested-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20240607035016.2975799-1-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>