hw/char/pl011: Pad PL011State struct to same size as Rust impl

We have some users of the PL011 struct which embed it directly into
their own state structs. This means that the Rust version of the
device must have a state struct that is the same size or smaller
than the C struct.

In commit 9b64209 ("rust: pl011: switch to safe chardev operation")
the Rust PL011 state struct changed from having a bindings::CharBackend
to a chardev::CharBackend, which made it grow larger than the C
version. This results in an assertion at startup when QEMU was
built with Rust enabled:

 $ qemu-system-arm -M raspi2b -display none
 ERROR:../../qom/object.c:562:object_initialize_with_type: assertion
 failed: (size >= type->instance_size)

The long-term better approach to this problem would be to move
our C device code patterns away from "embed a struct" and (back)
to "have a pointer to the device", so we can make the C PL011State
struct a private implementation detail rather than exposed to
its users.

For the short term, add a padding field at the end of the C struct
so it's big enough that the Rust state struct can fit.

Fixes: 9b64209 ("rust: pl011: switch to safe chardev operation")
Reviewed-by: Zhao Liu <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Bonzini <[email protected]>

Author: pm215

include/hw/char/pl011.h

@@ -52,6 +52,11 @@ struct PL011State {
     Clock *clk;
     bool migrate_clk;
     const unsigned char *id;
+    /*
+     * Since some users embed this struct directly, we must
+     * ensure that the C struct is at least as big as the Rust one.
+     */
+    uint8_t padding_for_rust[16];
 };
 
 DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev *chr);