Update `load_cmdline` to use `Cmdline` instead of CStr 

The [load_cmdline](https://github.com/rust-vmm/linux-loader/blob/4ab9dade2cbe6cf2c8707d386aa3fa4e3528511e/src/loader/mod.rs#L207) function should take as parameter the crate define [`Cmdline`](https://github.com/rust-vmm/linux-loader/blob/4ab9dade2cbe6cf2c8707d386aa3fa4e3528511e/src/cmdline/mod.rs#L99) abstraction.