gdbstub: change GDBState.last_packet to GByteArray

gdbstub: change GDBState.last_packet to GByteArray

Remove the packet size upper limit by using a GByteArray
instead of a statically allocated array for last_packet.
Thus we can now send big packets.

Also remove the last_packet_len field and use last_packet->len
instead.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191211160514.58373-2-damien.hedde@greensocs.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200316172155.971-27-alex.bennee@linaro.org>