be-hci: use backend functions

Avoid accessing CharBackend directly, use qemu_chr_be_* methods instead.

be->chr_read should exists if qemu_chr_be_can_write() is true.

(use qemu_chr_be_write(), _impl() bypasses replay)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Andrzej Zaborowski <balrogg@gmail.com>
