We got a bug report indicating that gdbserver --multi didn't work on
MIPS. We'd connect to the target, try to read the XML description -
gdbserver reports an error because there's no process yet - and then
try to guess the description from the g packet length instead. Of
course we can't do that; there's no process yet.
This patch fixes the problem. We try qXfer on connection, then try
'g' only if (A) there was no XML description, and (B) there is a
running process, and (C) we are not in non-stop mode; assume that
non-stop targets can provide a description explicitly.