run.py: Use 'signal' option of chardev to control signal behavior

The '-nographic' option is deprecated and no longer imposes
the behavior which we want since this commit:

qemu.git 02c4bdf1d2ca8c02a9bae16398f260b5c08d08bf

We should use 'signal' option of the chardev instead, which
works with current qemu master as well as on 1.4.0

This patch renames '-g' option to '-s' as the former no longer
has adequate name.

Reported-by: Juan Antonio Osorio Robles <jaosorior@gmail.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>