gdbstub: Fix client Ctrl-C handling

gdbstub: Fix client Ctrl-C handling

The gdb remote protocol has a special interrupt character (0x03) that is
transmitted outside the regular packet processing, and represents a
Ctrl-C pressed in the client. Despite not being a regular packet, it
does expect a regular stop response if the stub successfully stops the
running program.

See: https://sourceware.org/gdb/onlinedocs/gdb/Interrupts.html

Inhibiting the stop reply packet can lead to gdb client hang. So permit
a stop response when receiving a character from gdb that stops the vm.
Additionally, add a warning if that was not a 0x03 character, because
the gdb session is likely to end up getting confused if this happens.

Cc: qemu-stable@nongnu.org
Fixes: 758370052fb ("gdbstub: only send stop-reply packets when allowed to")
Reported-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Joel Stanley <joel@jms.id.au>
Message-id: 20230711085903.304496-1-npiggin@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>