Xfer:features:read truncating xml sent to gdb frontends

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1703147
Reported by 'Duane Voth' on 2017-07-08 :

Around line 1326 in gdbstub.c:

            if (len > (MAX_PACKET_LENGTH - 5) / 2)
                len = (MAX_PACKET_LENGTH - 5) / 2;

is truncating processor reg description xml files longer than 2045 bytes.
Deleting these lines works for my immediate need, but they seem to be
trying to fix some buffer overrun condition so I won't offer a patch until
we understand their purpose.