gdbstub: Do not kill target in system emulation mode

Too many VM kittens were killed since 7d03f82f. Another one just died
under my fat fingers.

When you quit a kgdb session, does the Linux kernel power off? Or when
you terminate gdb attached to a hardware debugger, does your board
vanish in space? No.

So let's stop terminating QEMU when the gdbstub receives a kill commando
in system emulation mode. Real termination can still be achieved via
"monitor quit". We keep the behavior for user mode emulation which is
arguably more like a gdbserver scenario.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>