Implement WriteVolatile Trait for SerialBuffer to avoid using intermediate buffer

[likebreath]

See #5949 (comment):

There has been quite many changes to the depend crates, particularly the newly added WriteVolatile/ReadVolatile Traits to the vm-memory crate along with deprecated APIs. For now, I am using an intermediate buffer for custom structs that does not have WriteVolatile/ReadVolatile traits implemented so that we can migrate from the deprecated APIs. There will be a performance penalty introduced from using an intermediate buffer (which is basically what's done from the deprecated APIs).

I believe this is fine for block::qcow::{Rawfile, QcowFile} (being used from block::Request::execute()), as the it will only impact vhost_user_block (while implementing the new traits for Rawfile and QcowFile seems to be much more complicated). The only other struct is SerialBuffer (used by virtio_devices::console::ConsoleEpollHandler::process_output_queue(), and I will take look into how to implement the WriteVolatile trait for it.