gdbstub: allow killing QEMU via vKill command

gdbstub: allow killing QEMU via vKill command

With multiprocess extensions gdb uses 'vKill' packet instead of 'k' to
kill the inferior. Handle 'vKill' the same way 'k' was handled in the
presence of single process.

Fixes: 7cf48f6752e5 ("gdbstub: add multiprocess support to
(f|s)ThreadInfo and ThreadExtraInfo")

Cc: Luc Michel <luc.michel@greensocs.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Tested-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-id: 20190130192403.13754-1-jcmvbkbc@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>