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: 7cf48f67 ("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>