rust: chardev: provide basic bindings to character devices

rust: chardev: provide basic bindings to character devices

Most of the character device API is pretty simple, with "0 or -errno"
or "number of bytes or -errno" as the convention for return codes.
Add safe wrappers for the API to the CharBackend bindgen-generated
struct.

The API is not complete, but it covers the parts that are used
by the PL011 device, plus qemu_chr_fe_write which is needed to
implement the standard library Write trait.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>