target/riscv/kvm: add software breakpoints support

target/riscv/kvm: add software breakpoints support

This patch implements insert/remove software breakpoint process.

For RISC-V, GDB treats single-step similarly to breakpoint: add a
breakpoint at the next step address, then continue. So this also
works for single-step debugging.

Implement kvm_arch_update_guest_debug(): Set the control flag
when there are active breakpoints. This will help KVM to know
the status in the userspace.

Add some stubs which are necessary for building, and will be
implemented later.

Signed-off-by: Chao Du <duchao@eswincomputing.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240606014501.20763-2-duchao@eswincomputing.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>