gdbstub: fix vCont packet handling when no thread is specified

gdbstub: fix vCont packet handling when no thread is specified

The vCont packet accepts a series of actions, each being applied on a
given thread ID. Giving no thread ID for an action is valid and means
"all threads".

This commit fixes vCont packets being incorrectly rejected when no
thread ID was given for an action.

In multiprocess mode, the GDB Remote Protocol specification is unclear
on what "all threads" means. We choose to apply the action on all
threads of all attached processes.

This commit is based on the initial fix by Lucien Murray-Pitts.

Fixes: e40e5204af8388
Reported-by: Lucien Murray-Pitts <lucienmp_antispam@yahoo.com>
Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190325110452.6756-1-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>