rust: pl011: Really use RX FIFO depth

rust: pl011: Really use RX FIFO depth

While we model a 16-elements RX FIFO since the PL011 model was
introduced in commit cdbdb648b7c ("ARM Versatile Platform Baseboard
emulation"), we only read 1 char at a time!

Have can_receive() return how many elements are available, and use that
in receive().

This is the Rust version of commit 3e0f118f825 ("hw/char/pl011: Really
use RX FIFO depth"); but it also adds back a comment that is present
in commit f576e0733cc ("hw/char/pl011: Add support for loopback") and
absent in the Rust code.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>